It all depends on what colors your fish are, and of course what you prefer. If you have bright tangs, and butterfly fishes, I think black makes them look awesome....especially if you have really colorful corals. But if you have the more monotoned colored fish like some angels, triggers, or lionfishes, then that really vibrant blue color brings out the detail in their coloring. I thought of a mirror before...I would love to see a picture of that!!!
I don't know, if the fish are half as vain as some of us are, they'd love the mirror!!!