By "black light" I assume you mean a UV light. I haven't read any studies about the effect of UV on fish over the long term, but consider that this is the same wavelength of light used in UV filters.
How do UV filters work? Bacteria, viruses, etc. passing through the filter are KILLED by the light. Now, the level of damage caused to living organisms by UV light is dependent on the strength of the light, promixity to it and dwell time under the light (hence higher wattages required for bigger tanks), so I don't think you'll see fish going belly-up all of a sudden. Nevertheless, I would be wary of long-term low-level exposure especially if you plan on leaving the lights on all night each night. Then there's consideration of how UV would affect your eyes if you spend a lot of time staring into the tank (I don't know the effect, but I would look into it).
Anyway, you can DIY your own moonlights for cheap from a couple of blue LEDs or use actinics. Heck, I've even heard of someone using a string of blue LED X'mas lights as moonlights. And these are completely safe.
