According to the Electrical Fun website, black lights are built using either an incandescent lamp with a special coating or a fluorescent lamp containing a special phosphor. Although the incandescent lamps create more visible purplish glow, the phosphor-enriched fluorescent lamps are more efficient in producing the ultraviolet (invisible) light that makes some objects glow in the dark.
When high-energy ultraviolet photons (light) emitted by a black light strike a black light-reactive (fluorescent) material, some of their energy is absorbed by the material and longer wavelength visible light is emitted, producing a glowing effect. When the light is switched off, the effect ceases. However, when phosphorescent materials are exposed to ultraviolet energy from a black light, they absorb some of the energy and can continue to glow well after the light is switched off.
White clothing often exhibits phosphorescence when exposed to black light because of the phosphors found in many laundry detergents. Some organic items such as crystals, strontium aluminate and zinc sulfide also exhibit phosphorescence under black light.