First off, there's a world of difference between a UV light used for reptiles to allow them to produce vitamin D and a blacklight.
Second, the UV lights used for reptiles are bright enough that they'll disturb the reptile if used at night.
While you'd be okay using a reptile UV light on your snake(s?) during the day, the effects of using a normal blacklight at night are still unknown.
I do agree with what was previously stated about blacklights. I recall hearing that they're bad for snakes, but can't remember who said it or what the reasons were, which is why I advise you to continue to research the subject and NOT install one until you know about the health effects of the particular variety you intend on using.
-Kat
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"You keep WHAT in your freezer?"
"Mice. And rats. If that bothers you, I can call them 'cows' instead."