From what I understand fish are very sensitive to light from the blue end of the spectrum, so even though it doesn't look bright to us the black lights (I'm told, please chime in at any time here
) will seem very bright to the fish.
That being said, fish under a black light are cool to watch. At night (in my FO tank) I will turn off the daylight bulb and turn on my black light. To diffuse the light I have the black light mounted at the bottom of the stand, pointing up. This diffuses the light and causes more of a "glow" rather than having the light shine down directly. I also have some of the new laser lights (from petco) to add a few "spot light" effects. (I know, I know, sounds cheesey, but the effect is rather cool)
Once I turn on my "nightlight" most of the fish will go to sleep and the nightlife begins. The extra light doesn't deter the crab and shrimp from coming out, the tank looks like a whole new world. . .
Instead of just building a reef in my home...I so wish I could afford to build my home in the reef! 
