I'd allow this effect. You couldn't create real darkness, but you could make a visual illusion inside a naturally dark area.
If you have darkvision, you can cast illusions normally in a dark area. They'll be visible to other people with darkvision, and if anyone turns on the lights, they'll be visible to normal sight as well.
If you don't have darkvision yourself, just light the room with torches, and then put the torches out after you've created the image. In a pinch, I'd allow you to cast a figment even if you couldn't see what you were doing (in magical darkness for instance), but anyone seeing it would get a hefty bonus to their disbelief roll.