There is enough human waste in orbit that the returning shuttle and the exterior of the Space Station are "krudding" up! This is true though, and the astronauts have to be careful sanitizing themselves after a space walk. Frozen bits also can act as meteors striking with enough force to injure an astronaut. Kind of like a frozen brown mist in space!
Don't you think the dark side of the moon would be dark enough?
You can't be guaranteed that it will stay dark.
First, even if it is very dark on the dark side of the moon, I wouldn't say that the "dark side" is totally dark. Just dark-er. Then you would have to deal with reflected light, star light and light from comets or other heavenly objects.
Finally, how would you deal with some idiot who might walk up with a flashlight and say, "Hey! What are you doing over there?"
That all begs the question that you need a clean area to work in. You'd have to build an enclosure anyway. Therefore, you'd just as well build your darkroom wherever it's most convenient. If you're going to go through all the trouble to build a darkroom, it would be silly if you built it out of glass.