a black light is called that because of the light it puts out. if you know anything much about light...not lighting...then this will be easier to understand
ok first things first there are many colors in one single beam of white light...there is all the colors of a rainbow. if you have ever used a prism then you understand this concept.
now, the way we get different colors is by the color.say we have a green wall, it absorbs all the colors except for one...green. the green light gets reflected back at our eyes and we see green.
now with a black light its is a bit trickier. when you have flourescent black lights it is a normal flourescent light except the tube has a coating that filters our most of the visible light and only lets UV-A light that makes the cool colors. the UV-A light is reflected off of phosphors on things like ur cloths teeth and other stuff. black cloths don't "glow" because black absorbs all light.
to simplify...blacklights only output UV-A color which reflects off of phosphors and then we see a "glow". it doesn't work an all materials because they don't reflect UV-A...they abosorb it.
sorry i wrote kinda long. if ya want more info go here:
http://science.howstuffworks.com/black-light.htm DJ Borris