I have been to a couple of bars that had darkrooms/backrooms.
The first one was many years ago, it was Faces in East St. Louis, IL. This bars in St Louis, MO, closed early and this one in East St Louis would stay open until late (4am?). The basement of this bar was a men's only bar that would stay open even later. I remember leaving the basement bar at 8am a few times. In the back corner was the bathroom and a darkroom. The only light in the room was an exit sign, and it usually got covered up with somebody's shirt so the room would be pitch black. On weekends, it was wall to wall men in this pitch black room. Pretty much anything and everything sexual you can think of going on around you and your only way to get around/know what was around you was to feel you way around. A few years later, they moved it to a space in the middle of the basement bar with chain link fencing as the walls, so it wasn't as dark. Still fun, but now you were on exhibit in front of everybody down there. Not sure if this place is still open, last time I was there was 20+ years ago.
The other one that comes to mind is the Phoenix in New Orleans. The upstairs in pretty dark (the bartenders use lighters to see in the beer cooler). I've seen stuff happen in the main room upstairs, but the fun room is the room off the main room. The only light is a single lightbulb located inside an air vent. It gives off a little light, but not a lot. I've sucked, been sucked and been fucked up there a few times. On busy nights, it can be wall-to-wall men in that room. Fun times. Once again, I haven't been there in 20 years, not sure if it is still open or the dark 2nd floor is still run that way.