UV Reactive paint is nothing special. It is the genius behind the design of Black Lights that is interesting.
Black lights look just like normal fluorescent lamps or incandescent light bulbs, but they do something completely different. Switch one on, and white clothes, teeth and various other things glow in the dark, while the bulb itself only emits faint purple light.
The conventional black light design is just a fluorescent lamp with a couple of important modifications. Fluorescent lamps generate light by passing electricity through a tube filled with inert gas and a small amount of mercury.
When energized, mercury atoms emit energy in the form of light photons. They emit some visible light photons, but mostly they emit photons in the ultraviolet (UV) wavelength range. UV light waves are too short for us to see -- they are completely invisible -- so fluorescent lamps have to convert this energy into visible light. They do this with a phosphor coating around the outside of the tube.
Phosphors are substances that give off light -- or fluoresce -- when they are exposed to light. When a photon hits a phosphor atom, one of the phosphor's electrons jumps to a higher energy level, causing the atom to vibrate and create heat. When the electron falls back to its normal level, it releases energy in the form of another photon. This photon has less energy than the original photon, because some energy was lost as heat. In a fluorescent lamp, the emitted light is in the visible spectrum -- the phosphor gives off white light we can see.
The "Black Coating" of Black Lights absorb the harmful shortwave UV rays B and C. The "black" glass tube itself blocks most visible light, so in the end only benign long-wave UV-A light and some blue and violet visible light pass through.
All in all it is the Phosphors within the UV Reactive Paint that reacts with the UV-A rays and emit a glow of purple.