I wouldn't hang a normal black light over a fish tank. I'd buy a moonlight specially designed for aquaria. I'm not sure if a regular blacklight's spectrum would be safe for freshwaster fish. (But I may be wrong.) Or, if you want a bright tank with blue hues, get an actinic bulb - but remember that with an actinic bulb over a freshwater tank, you'll get a lot of algae.
Moonlights in a freshwater tank allow you to see your fish after the regular tank lights are turned out. They neither help nor hinder your fish. In a saltwater tank I understand they are beneficial.
I'll do some checking about the black light as far as being harmful is concerned but I really don't think it woud be helpful at all.
Black lights are completely safe. They are just standard fluorescents with a special coating that blocks out certain wavelengths of light. All fluorescents including black lights give off relatively harmless UV-C radiation so that shouldn't be a reason for concern. Black lights do make certain colors glow very bright which looks cool but not natural.