I stayed three nights in Hotel Black Cologne. The hotel is located in Ehrenfeld, which for those who don't know the Cologne area, is quite a way away from Cologne centre.
The hotel is striking and the main lobby is pretty and enticing in the all black manner. The corridors are long and sleek. My room was however none of the above adjectives. It seemed as if the room was in the middle of undergoing a restoration; the walls didn't match, there was visible piping coming out of the bathroom ceiling and a smell of stagnant water was constantly unpleasant. The bath felt dirty and paint was coming off in flakes from the walls. The room door handle kept getting stuck too making it even difficult to escape. The room was unbelievably noisy with the studio above.
The bed was despite this very comfortable and the staff very pleasant.
Pros: The staff, the bedding and the idea.
Cons: Execution of the idea; removing/hiding bathroom piping, getting rid of the stagnant water smell, fixing the door handle.
Location: There is a nearby U Bahn but be prepared to spend plenty on train tickets in and out. Ehrenfeld is out of the way with many Turkish restaurants.