"Black Gold" is an achievement you get for finding all 7 gas stations in Valo City. While it's not a very hard task (especially compared to other achivements in this game), I know that some would rather have it all lined out in a guide, so here goes.
Apart from the achievement, of course, the gas stations come in useful during normal gameplay, as they serve as a way to refill your NOS without heading back to the garage. If you didn't know that yet, now you do and you're welcome.

The gas stations are scattered all over Valo City, so obviously you'll have to unlock all parts of town before you can unlock the achievement. That being said, nothing's stopping you from looking for the gas stations in the districts you currently have access to.
Also, this is more of a tip but I suggest you pick a car with a bit of ground clearance for your "discovery mission", as some of the gas stations (notably the one in eastern West Side) don't have a proper entrance, requiring you to hop a curb to access the station itself.
Speaking of access, you don't have to actually get a refill at the station to mark it as "discovered"; just drive into the pump area and the words "Gas Station Found" will pop up on your screen, along with it being marked on your map.

Gas stations by district:

• 2 in Southern District, one in the west and one in the far southeast
  
• 2 in North East, one in the southwest and one in the central area (right next to the parts shop)
  
• 1 in North End, in the east
  
• 2 in West Side, one in the far south and another in the east

So yeah, here's the list and locations of all 7 gas stations, whether you need them for gameplay or if you're just achievement hunting. Whichever may be the case, hope this helped, thanks for reading and happy driving!