then they can definately look up at a black sky and see a bright glowing purple light that signifies things are out of the ordinary and bigger fish shut down.
They could just as easily look up and see a big dark shape(aka boat) going across as well, would almost make sense that the bottom of boats would be better painted white for night fishing.

I don't use a black light at night but if I did I would mount in on the inside of the boat so the main glow wouldn't be visible below. I would think that most fish would see them enough though that they should be pretty acclimated to them.

Scientific reasonings or not if you aren't catching fish I would just change your technique(s) since there are people on both side of the fence but the fact is people still catch fish at night with black lights visible to the fish, it just depends on how anal you want to be about the lights or how much you want to use that as an excuse when you don't catch fish