To reveal the secret contents of the Weasley twins' infamous "Marauder's Map," a wizard had to tap it with a wand and say, "I solemnly swear that I am up to no good."
Obviously, Potterheads are drooling over the sneaky ink, but other commenters are bringing up the safety of black light tattoos in general. Because the FDA "has not approved any tattoo pigments for injection into the skin," it's not really regulated. It might depend on your skin as to what kind of side effects develop.
If you're thinking about getting one yourself, you should probably ask an expert about the health implications. If you just want to ogle a Harry Potter fan who's gone farther than you ever will, then stay right where you are. You're in the right place.