When my tank was first set up, I had a blacklight as a night light. I added a scattering of "flourescent" colored gravel and some white silk plants. The effect at night was really cool...for a while. Over time, I realised that though some aspects of the scenery looked great under black light. None of the fish "reflected" the black light. Outside of the white plants and glowie gravel.... everything else was dark and shadowy.. especially the fish. I got bored witht the blacklight and switched to a "moonlight" setup. This allows for much better night time viewing.
For what it's worth, in about 2 months I saw no ill effects from the blacklight. A trusted LFS
owner, when I asked him about blacklight, said that the only fish that he had seen affected by a blacklight were knifefish. He said it seems to mess up thier sense of direction or something like that. (coughurbanlegendcough)