For the achievement i just fell into the endless void room using a blue key on one of the doors, it seemed to work for me that way
I didn't find an endless void room anywhere, just the door that warped me back to the start of the level. Even if I did pass by it, it was probably one of those perma-locked doors that wouldn't open for me, regardless of what color key I used.
There should be a door somewhere in there that you use the blue key on that takes you to a endless pit.
