I threw this map together as a reference for the main four floors in the Black Zone.
There's also a Floor D, but that's really only one room so I'll put that map when we get to it.
For now, use this for reference if you find yourself lost in the main floors.

Before we enter the Black Zone, we're going to need close to a full inventory. It's a good idea to leave a space free for some Diskettes we'll be nabbing on our way up to the Power Room.

Here's what we need to bring:
At least one weapon
The Night Visor
The Screwdriver
A Wi-Fi Receiver
The Wi-Fi Remote
The Power Node

Not required but always beneficial is the Body Armour, Medkits and the Crowbar.
Once you're ready, exit the Save Point in the Red Zone and enter the door to your left to head into the Black Zone.

Floor A
As you can see from the map above, we entered into Floor A. Before, we headed straight forwards into Floor G, but our current plan is to open up Floor C and climb up to Floor D.

From the entrance, turn right and head down the corridor.

At the end, there will be a crate you can Crowbar open containing 2 Diskettes. Then, head through the door to the left.

Lift

The Lift is a platform you can climb up to move to the floor above you. When you power up a certain floor, and the Lift itself, you can move the Lift to a specific floor.
Climb up onto the boxes conveniently stacked up on the Lift.

From there, you can see the door behind and above you. Simply grab the ledge, shimmy around to under the door, and pull yourself up to enter into the next floor.

Floor B

Directly in front of you is a door, but ignore that for now. We won't need it for a while. Slowly walk down the corridor to your right and be ready to fight.

On the next right turn, a Walker stands waiting right around the corner. It'll attack as soon as you approach, but as long as you're quick to strafe around and headshot it, you should be fine.

Head down the corridor the Walker was standing in and enter the door.

Stairway
The Stairway is another way floors are connected in the Black Zone. There are crates blocking your path between A and B, but you can use the stairway to freely cross between Floors G and A, and Floors B and C.

You might also notice that the lights are on in the Stairway. We'll change that, don't worry. Walk upstairs and enter into the next floor up.

Floor C
Head straight forward to approach a locked door and some crates.

Hop up onto those crates and to the left you'll find a vent to crawl into the next room.

C-Power Connector

Exiting the vent, drop from the crates and enter the door on your right.

Power Room
Here you'll see a locked gate with two buttons - one on either side.

You need to hit both buttons at once. It's possible to do simply by being really fast but save yourself the time and do the smart thing - use the Wi-Fi Receiver.
Simply slap a Receiver onto one of the buttons, click the Remote and right as you get that beep, press the other button.

This here is the power board. By plugging one of your Power Nodes under a certain letter, you will power a part of the Black Zone:

Floors A, B and C are powered by A, B and C, sensibly enough.
L powers the Lift. To move the Lift to a floor, you need have both that floor and the Lift powered.
S powers the Stairway. You never need to have the Stairway powered, ever, for any reason, so it being here is a waste.
Take the Power Node out of S - now we have two. Plug them into B and L to power up Floor B and the Lift.

Oh, and don't forget to grab your Receiver again. Next, we move down to floor B.

C-Power Connector

Back up into the vent.

Floor C

And back into the Stairway. As you can see, this place isn't as confusing as it might seem at first.

Stairway
Lights are off here now. Simply head down back into Floor B.

Floor B

A Slasher is in play now, just around the right corner. Kill it, and then head into the door behind it. It was locked before, but when B is powered, you can make use of the room beyond.

B Save Point

Use the Item Box in here to store your Diskettes and resupply your ammo stacks. As long as you've got plenty of Diskettes, you may as well save too.
Exit back out.

Floor B
Heading back down the corridor, there's another Slasher on your right.

This one, however, you can actually line up a headshot before it even sees you. Just horizontally line it up and slowwwly scoot to the left. When your crosshair turns white that means you're lined up and the wall isn't blocking your shot. Point your sights directly at the Slasher's juicebox and unload.

To your left, across from the Slasher, is the Lift. Don't enter the Lift - simply hit the button to the right of it. You should hear a metallic clunk of the Lift moving up to B.
We do need to climb up to C via the lift, but we need to power C first. You know what that means - back up the stairs. Turn around and back the way you came, right around the corner and back into the Stairway.

Stairway
Back up to C. Be ready to fight.

Floor C

A Walker approaches in the dark in front of you. Trivial. Kill it and return to the vent behind it.

C-Power Connector
Back into the door on the right.

Power Room

Switch your Power Node to have C and L powered. Then head back out.

C-Power Connector

Rather than hopping through the vent, head around the left. Now that C is powered, you can open this door with the button next to it. Neat. Head on through.

Floor C

A Slasher waits around the right corner. Should be no problem. Head back into the Stairway...

Stairway
Back down into B...

Floor B

And back to the Lift in B. This time, head through it for real.

Lift

Climb up onto the floor above to return to Floor C via the Lift.

Floor C

Around the corner on your right, a Walker guards a button. Kill it, and then hit the button next to it to open up a gate. This opens your way back to the Power Room without needing to faff about between B and C. Once opened, turn right back around and return to the Lift.

Before you enter, hit the button next to it to move the Lift up to Floor C. Then enter.

Lift
As before, climb up onto the crates and grab the ledge to pull yourself up into the darkness. Then, we can enter Floor D.

Floor D


In this room, a Wi-Fi Receiver can be found on the opposite wall directly ahead of you. A Walker stands in front of it, facing it. To your left is a Slasher, and around the corner on the right, a second Slasher.

The Walker won't attack, not unless you give it a reason to, so ignore it and focus on the two Slashers.

As soon as one Slasher sees you, both will attack - you've done this before. Strafe out and kill the Slasher on your left as quick as you can. The second one will sprint through the middle of the room, so spin around and catch it off guard.

If you fail, don't be afraid to use a Mine for the retry. We have plenty to spare.

The Wi-Fi Receiver can be snatched from in front of the Walker's face, sitting around on a button. As long as you don't shoot the Walker, or walk directly in front of it, it won't have any clue you're even there.
You could just kill it anyway, if you've got the ammo to spare. Up to you.

Oh, and the button the Receiver is on? Turns the lights off in this room. As far as I know, it serves no real purpose. Maybe it opens a secret room somewhere with a rocket launcher in it. Probably not.

Now that you've nabbed two Wi-Fi Receivers, we can start cracking on. Before we start working on Keycards, there's one thing we absolutely need to do first.

Lift
Exit into the Lift and drop down the crates to return to Floor C.

Floor C

Head around the right past where the Walker was, and into the door on your left to head back through the C-Power Connector into the Power Room again.

Power Room

Switch your Power Node from C to A, powering up Floor A and the Lift. It's a bit of a detour, but it's a bit of a necessity.

Leave the Power Room, and return into Floor C, down the Stairway, into Floor B, and from there enter the Lift.


Lift
The Lift is above us, so we'll be making a drop of faith. Nothing scary about it, don't worry.
From the ledge in front of you, turn and hang down from it. Then, simply drop off the ledge and don't touch any keys - if you hold forward, you might land on top of the ledge below and die. Haydee will automatically grab the ledge down at Floor A.


At the very bottom you can loot a Haydee corpse for a Medkit. Then, exit into Floor A.

Floor A

As soon as you enter, turn to your left and hit the button. If Floor A and the Lift are powered, this will drop the lift back down to A.
That's all we needed to do. Why? Because the stairway between A and B is blocked - if you dropped down to A with the Lift still up at C, you'd have no way of getting the Lift back down here... and you'd be stuck out of the upper floors of the Black Zone, softlocking your game and rendering it unwinnable.
If you screwed up and didn't power Floor A and the Lift, reload your save immediately.

Otherwise, you won't be able to get the two keycards in this part and if you overwrite your save you'd be forced to start the whole game from the beginning.

Now that you've understood the importance of what we've just did - no need to worry, because as long as the Lift is at A, you'll always be able to climb back up to B and C to the Power Room.
Let's do exactly that, because we need to grab one of the Nodes.

Turn back around into the Lift, climb up to Floor B, head through the hall into the Stairway, walk up a floor to Floor C, straight ahead and through the door to the C-Power Connector, and finally back into the Power Room.

Quite a trek, but at least I don't have to screenshot every single room - hopefully you know your way between floors. If not, use the map at the top of the guide.
Once you're back in the Power Room, take a Power Node out and pocket it. We need this to power up Floor G elsewhere. Pop your other into C.
With Floor C powered and the other Power Node in your inventory, we can head towards our first keycard in the Black Zone.

Floor C

Head down your hallway on your right and past the open gate to a door we haven't been through yet. This will lead into a series of rooms, a one-way loop of sorts.

Room C-1

Unnerving.

Room C-2

It's like a storage room for crazed swordbots. Jeez.
Ahead of us is a locked door. Where we actually want to go is beneath us. If you hang from the ledge on your right...

You'll see a gap you can drop down to. Just like the Lift drop previously, don't hold any keys while you're falling or you may take damage. Simply drop, grab, and pull yourself up.

On the lower level, head through the door on your right after pulling yourself up.

C-Keycard Room

Hitting the button in front of you opens up a cage containing a Red Keycard, accessed from elsewhere, but also locks the door behind you. This seems familiar somehow. Wi-Fi Receiver time!
Slap it on and return to the door behind you.

Room C-2
Head straight forwards into the room opposite.

Room C-3

Climb up onto the crates to your right and jump across the room to grab a ledge underneath some pipes.

From there shimmy around right, stand on the crates to jump up to grab the top crate and keep heading right.

On top of the crate stack you can find 20 Pistol Rounds. If you're wondering, no, the game doesn't ever stop giving you Pistol ammo - there's an achievement for beating the whole game with only the Pistol.

Once you've shimmied around to the top, there's a little crawlspace you can climb into. Be careful - there's a hard drop only a few feet into it.

Hang down from the ledge on the other side of the crawlspace and shimmy left to drop down onto some crates.

The vertical crate on the left side of the stack contains 2 Diskettes.

Be ready to fight and head through the door - the next room has a Slasher ambush.

Room C-4
Yikes.

Once you've taken a few steps forward, about halfway to that dead Haydee on the ground, two Slashers will spawn and attack - one ahead of you, at the end of the room, and one directly behind you.

Turn around and face the door you just entered from. Aim your weapon and start walking backwards - that way you can be ready to kill the closer Slasher immediately. Then, whirl around and stagger the second Slasher.

The aforementioned Haydee corpse has a Medkit. A Save Point is coming up, which is an optional detour but lets us restock and save.

Ahead, the corridor splits in two. We want the door directly ahead of us. Head through.

C-Save Connector
This room is safe, but has some tricky jumps where failure can be fatal. Be careful maneuvering upwards in this room.

First, however, if we drop down onto the grates, down onto the floor and then turn to our left, we can find a gap in the crates leading to a Haydee corpse clutching 40 SMG Rounds. Then, climb back up onto the crates.

Jump onto the lower crate to your left and then climb up to return to where the entrance is.

Climb onto the crates to the left of the door and move left back into the dark. Then climb up the crates to your right.

Make a good long jump onto the stack of crates in the corner of the room.
Turn to your left again to see some crates. While crouching, sprint from your perch down onto the crates to reach the door.

The top crate on the right side of the door contains 10 Magnum Rounds. Score. Enter the door.

C Save Point

Restock, resupply, heal if need be. Drop your Screwdriver into the Item Box too, we won't need it but we will need at least one inventory space for the keycards.

When you're ready to go, leave the save point room and head back.

C-Save Connector
If you're feeling particularly daring you can jump clear across the room to save some time, but take the safe way for now.

Jump back to the corner crates, sprint to grab onto the indicated crates and then work your way back down to the door. Be ready to fight - a Walker will be waiting just to your right as you walk in, and will attack as soon as you enter.

Room C-4
Ah, here it is.

Dispose of it and move into the next room.

C-Keycard Room
Once you enter, click your Wi-Fi remote. Don't approach the other side of the room.

Clicking up your Wi-Fi Remote will open up this Hell in a Cell cage match between our stacked android protaganist and four Walkers.

See those two strips of red lights along each of the walls? That's a movement detector - if you cross that red line along the ground, the lights are going to turn off and all four Walkers will be released and attack. You'll be caught in the middle of them and almost certainly die.

While it is possible to win that fight, particularly if you have Mines, we'll be avoiding it completely.

Enter the cage but don't cross it yet. When you're inside, click your Wi-Fi Remote a second time to turn off both the lights and the movement detector.

With the room dark, walk over to the other side of the cage and pocket your first Red Keycard from the Black Zone, and third in total.
The two dead Haydees slumped up next to it have 10 Pistol Rounds and 2 Diskettes.

Click your Wi-Fi Remote a third time to turn the lights back on, and hit the button on the side of the keycard's pedalstal to open up the next room. Then, click your Wi-Fi Remote a fourth time, cross back to the other side of the room, and click it a fifth time to let yourself out.

Or, I guess you could just do it the hard way?

With any luck, you should have nabbed yourself a keycard and opened the next door without any four-on-one battles against Walkers. One more thing before you continue though:

Hit the Wi-Fi Receiver a sixth time. This will lock the cage and as you might remember, unlock the opposite entrance to the room, where we placed the Receiver.
You don't actually need to get the Receiver back - you can just get by without it, so this isn't required - but it will definitely save some time later on and will also net us an achievement near the very end, so it's probably a good idea.

So, return into the previous room. and we'll quickly backtrack to obtain our Receiver again.

If you really don't care, just skip ahead to the next section.

Room C-4

Turn to your right and head down the corridor.

Room C-3

Climb onto the crates and along the ledge back into the crawlspace, then hang from that ledge, drop to the lower ledge, and then safely through to the floor. Head through the door.

Room C-2
And keep going through the opposite door again.

C-Keycard Room

Snatch it, exit through Room C-2 and back into Room C-3.

Room C-3

The whirlwind of backtracking fun continues.

Room C-4
Head forwards and into the left door to return to the C-Keycard Room.

C-Keycard Room
Good to go? Okay. Head through the door opposite where you came in from.

Room C-5

Another room, another array of swordbots.

Hang down from the ledge immediately in front of you. Where we want to be is the little crawlspace a ways down this big bottomless pit we're over. Drop down onto the crate and crawl through.

Inside, you'll find a power module. Good thing you brought that Power Node with you, huh? Plug it in and you'll get a nice bright G indicator next to you - we just powered up Floor G and opened the way to get our next keycard.

Shimmy back to the crate, pull yourself on top of it and climb back up to where the door is.
I'm not sure having power modules necessary for distributing power through your facility in tiny little vents partially down bottomless pits is exactly OSHA compliant but...

Anyway, carry on to the right side of this room, up onto the ledge again and up the crates to your right.

If you turn around and jump to the crates behind you, you can snatch a Medkit lying atop them. Then, you can turn back around and jump to the railing.

After pulling yourself up onto the top ledge of this room, the crates on your left have a Haydee cadaver with 2 Diskettes atop them. Finally exit through the door to return to the beginning of the 'one way loop' of the C-Rooms.

Room C-2
We wash our hands of this mess and begin the hike down to floor G.

Room C-1
Exit back into the main hall of Floor C and get your gun ready.

Floor C

There's a Walker just around the right corner as you walk in.

And a little further forward, a Slasher down the left hallway. Dispose of both of them and head into the Power Room.

In the Power Room, switch your Power Node to B. This will let us access the Save Point and Item Box in Floor B again.
From there, exit back into Floor C and into the Stairway, then head down a floor.

Floor B
Take the right turn, into the save point.

B Save Point
Save your game and restock ammo if you need it. You can chuck your Crowbar, Wi-Fi Receiver(s) and Wi-Fi Remote into the box, we don't need them for a bit.

Bring your Body Armour, Medkits, Night Visor and a weapon or two.

I strongly recommend you bring at least two Mines, as an upcoming room is made much easier with them.

The more free spaces you can save, the better - there's plenty of loot ahead.
Ready? Exit back out and continue heading down the Black Zone.

Floor B
Continue forwards and left at the end of the corridor into the Lift.

Lift
Assuming you haven't made a massive mistake, the Lift is at A. Simply climb down the crates below you and exit. Be ready to fight!

Floor A
As soon as you enter Floor A, a Slasher is going to come charging down the corridor at you from the right.

If you're fast enough, you can just strafe out into the corridor and gun it down, or you can wait until it comes to you. Whichever works.
Head down the corridor and take the next turn right to head back into the Stairway.

Stairway
Climb down another floor to enter the ground floor. Keep your gun handy, the Slashers aren't going to let you get this keycard easily.

Floor G
Head down the corridor and take the left. Around the next right awaits a Slasher.

Killing it shouldn't be too hard. However, when you reach the door behind it, another Slasher will spawn at the entrance of this room. You probably won't even hear the drop vent from this far away. Run to the door, and turn around. Wait for the Slasher to charge around the corner before you move on.

There's a Haydee at your feet with 10 Pistol Rounds, too. With both Slashers scrapped, enter the door for real.

Room G-1

This room's safe.

Room G-2

This one isn't. Don't move - instead, take a quick look at this map:


Four Slashers, one in each corner, and all of them around tight angles that make it hard to pick them off. Sure enough, as soon as one sees you, all will attack.

Time to use our Mine!

Place your Mine right at your feet in front of the door and move to the middle of the room so you don't take yourself out with it.
This will take care of at least one Slasher and thin the herd. Your second Mine would be put to good use on the other side of the room, but it's important that you don't place it until you're ready to commit to a fight.
Once ready, place your mine just before the little T-section ahead, and then immediately run to your left to take out the first Slasher. You're going to want to stagger it as quickly as possible and then spin 180 degrees, killing the second Slasher rushing you from behind.

You may actually stagger yourself with the Mine, but as long as you're quick enough moving, you will take very little damage, if any at all. The other two Slashers will be taken out by your placed mines.

If you don't have a Mine, you'll need to point-blank peek a corner and whirl around to take the other three - this is a situation where having both weapons ready is a good idea.

Once you're done, you need to press the buttons behind where the Slashers spawned to turn on all four lights ahead.
Press the back-right button, the back-left button, and the front-left button (labelled as BR, BL and FL in the map above) to unlock the next room.

Room G-3

Hang from the ledge in front of you, shimmy around to one of the sides, and then drop to grab the ledge below you. Then shimmy back so you're directly underneath where you entered from, and head through the door.

G Keycard Room

...

A Walker stands in front of you. Kill it. From the Haydee corpses in the room you can find the following items, in roughly the order of most to least important:

The second-topmost corpse in front of the kiln has a Red Keycard.

The second-topmost corpse on the big pile in the corner has a Yellow Keycard.

The splayed out corpse just to the left has 2 Diskettes.

The corpse on the ground in front of the kiln, closest to the exit door, has a Medkit.

The corpse slumped against the wall in between the door and the corner pile has 10 Magnum Rounds.

The pair of legs sticking out the other side of the corner pile has 20 SMG Rounds.

Finally, the corpse on her side in front of the entrance door and the spread-eagle corpse on the left side of the kiln pile have 20 Pistol Rounds - 10 on each.

Plug in your Yellow Keycard to the reader next to the door and exit into...

Red Zone, Room A-2

Huh. Convenient shortcut. Time to head on back. Climb past the gates to your left and exit back towards the Red Zone's Save Hub.

Red Zone, Room A-1

You've been through this room once before, coming the other direction. Climb up the three platforms to your right and onto the pipes to drop back to the door and return to the A-B-C Connector.

You can save your game if you want, but there's another one coming up shortly.

A-B-C Connector
Continue into the C-D-E Connector. That's this door:

C-D-E Connector

Climb up the familiar series of platforms until you can drop down next to a gate. If it's locked, you can find the button just on the other side of the frame behind you:

Unlock the gate, jump back over and continue through it.

Room E-1

Press forward.

Room E-2
This room has four platforms, two of which will be up at any given time. You can jump across in five seconds with clever use of a Wi-Fi Receiver, but we'll take it the easy way.

First, hit the button in front of you to raise a platform. Jump on the platform, and then jump into the cell on your right. Hit the button inside.

Jump back onto the first platform, and back to your left to the first button. Hit it a second time, and then move to your left. Jump to the platform, and jump again to your left. Hit the button there as well.

This will raise another platform. Heading into the doorways on your left leads to a Save Point, and straight ahead of you is progress. Head into the Save Point.

E Save Point

Grab all four of your Red Keycards from the Item Box and load up on ammo, as there will be quite a bit of resistance coming back out. Save your game and exit back out.

Room E-2

Jump onto the left platform, and then left again into a door.

Blue Keycard Room

One gate, four Red Keycard Readers. Beep beep beep beep.
If you take a look at the ceiling, you might notice two drop vents above the entrance. Enter the cage.

The main purpose of getting all these keycards is simply to get this - our first of two Blue Keycards.

When you grab it, two Slashers are going to drop in behind you, so be ready to fight and take it.

Bullets don't penetrate, so if you're still not as confident with your headshots, be sure you stagger them both individually.

The button unlocks the door, which just locked behind you. The Haydee corpse has 10 Pistol Rounds. Exit and begin your run back to the Save Hub.

Room E-2

Jump across the two platforms to the other side, hit the button there, and then jump from the right platform back to the entrance. Be ready to fight.

Room E-1

A Walker waits. Should be no problem. Keep moving.

C-D-E Connector

The door back has been mysteriously locked, so we'll have to go around. Jump up to the frame across from you where the button that unlocked the gate was, and head into the door on your left.

Room D-1

Climb over the crates to your right and down into the gap below and through the door.

Room D-2

If you didn't pick up the Pliers from the Red Zone, there will be a Slasher to your right as you walk in. Kill it.

Additionally, you can find 10 Magnum Rounds and a Medkit in a crate next to the Slasher and on a Haydee corpse across from it, respectively.
Either way, continue.

Room D-3, the Crate Stack

There's a huge pile of crates that you can climb up or down. Up above you is the Pliers room. Below us loops back toward the Save Hub.

Jump to grab the crates in front of you and then start climbing down the left. Enter the door at the bottom.

B-D Connector
Be ready to fight - this room was empty if you passed through it before. This time, it has a few hostiles.

At the first turn, a Slasher awaits to your immediate right in the corner, and another a little ways down to your left. You can headshot the first before it even sees you, and then strafe out to shoot the second.
About when you reach where the second Slasher was standing, a third Slasher will spawn at the entrance behind you. Turn around and sloooowly walk backwards until you hear it drop and rush you.

Finally, a Walker patiently waits at the exit door for you to kill. Exit the room.

Room B-1

Be ready to fight and head through the door on your right.

A-B-C Connector

As soon as you enter, a Slasher will attack from above you. It'll have to run all the way to the stairs, down the stairs and around the corner though, so you have plenty of time to deadstop it and move on.

Climb the stairs and exit through the door across into the Save Hub, and then finally into the Save Point.
Our work finally rewards us with a Blue Keycard, which means we're finally done with the Red and Black Zones. In the next part, we'll be heading into the Yellow Zone to snatch two more Green Keycards, and then ducking into the Green Zone to get our second Blue Keycard.
With both Blue Keycards, we'll enter the final zone and finish the game.

There will be a Save Point pretty much immediately in the Yellow Zone, but if you have plenty of Diskettes you can save it here for now.

Once collected, we can head into the Yellow Zone.