Fluorescent black lights are just regular fluorescent lights except that the normal glass envelope of the bulb is replaced by a deep-bluish-purple glass called Wood's glass,
which blocks almost all visible light above 400 nm. As a result, only the near UV lines of the Mercury atoms in the bulb pass through the glass.
A black light may also be formed by using Wood's glass instead of clear glass for a common
incandescent bulb but these are very inefficient at producing UV light due to the blackbody nature of the incandescent light
filament. Incandescent UV bulbs, due to their inefficiency, may become dangerously hot during use.
Spectrum of "Black" Light Bulb