-A exposure from a florescent blacklight will no doubt damage your fishes eyes the same way it will damage your eyes. You are looking at the things lit up by the light, but keep in mind you are pointing it at the fish.
-A light from a common "poster blacklight" does little to sterilize anything, for that you need UV
But shining it on the fish, you may end up with blind fish, or other problems.
If it is an incandescent
"black light", then these are not really true UV
lights, and do even less damage, if any.