GUEST,Sooz(at work): Black light is another name for ultraviolet light. Pure ultraviolet light is invisible, by definition, but most devices that produce ultraviolet light also emit a small amount of visible light, causing them to appear purple, and not strictly black.

Some substances have the ability to absorb ultraviolet light and emit visible light. DayGlo® fluorescent pigments (etc.) and the "brighteners" that are added to some laundry detergents do this, as well as some natural substances.