The blue key is in episode 2 level 18 and surrounded by walls on all four sides making it look like it's unobtainable. But there's a secret part of the wall you can walk through.

When you pass the upper laser, walk towards the wall shown in the picture. Once you walked through, "The Passage" achievement will pop up and there you go, get the blue key and the respective achievement (I had to draw the key on the screenshot since I took it after picking up the key).

If you played through and are stressed by the idea that you'd have to pass 17 levels just for a useless key, but you urgently want to complete the game, DON'T WORRY. All progress is saved and you don't have to pick up all keys and key cards again. Simply walk through the previous 17 levels until you reach level 18.

Go to episode 1 level 1 and walk straight into the blue door behind you at the beginning.

You can pass the first level by walking inbetween the two lasers forth, back and forth again. Yeah, this one's a bit more difficult than most regular levels and definitely doable.

If this guide helped you on your way to completion, please consider giving it a thumbs up. Feel free to ask questions in the comment section down below.