as far as I know, black lights are neither sold in more than 20" tubes, nor are visible except when on white or another color that lights up by it, and are not waterproof.
given, blacklights would look cool and be very original, but unless you can find a way to make them condition-proof to survive under a vehicle, I tjhink you'd just wind up blowing lots of money replacing them.
I've seen all kinds of weird stuff here, one guy had a halogen foglight set under his s-10... apparently he failed to ask anyone he saw around town what they did and assumed thats what it was. I saw them when he went up a hill, aside from that it just looked like a really weird set of neon (incandescent kinda glow) I've seen people take neon tag rings and put them under their cars, I've even seen people with a tube oinb each side under the vehicles and no front/rear tubes at all.... I can't udnerstand how, if people want to go for that kind of look, they're too cheap to spend $150 for A REAL SET instead of trying to rig all this stuff. I tell ya heh, neon may not be a very original idea anymore, but its certainly better than what some of the people here have tried lol