A blacklight poster is a poster printed with inks which fluoresce

Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

 under black light

Black light

A black light, also referred to as a UV light, ultraviolet light, or Wood's lamp, is a lamp that emits ultraviolet radiation in the long-wave range, and little visible light...

.
The inks used contain phosphor

Phosphor

A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...

s which cause them to glow when exposed to the ultra violet light emitted from black lights.

In the United States, blacklight posters are commonly found in head shop

Head shop

A head shop is a retail outlet specializing in drug paraphernalia used for consumption of cannabis, other recreational drugs, legal highs, legal party powders and New Age herbs, as well as counterculture art, magazines, music, clothing, and home decor; some head shops also sell oddities, such as...

s and other retail outlets that sell items associated with counter culture. Blacklight posters commonly depict images of psychedelia, musicians/bands, and hippie

Hippie

The hippie subculture was originally a youth movement that arose in the United States during the mid-1960s and spread to other countries around the world. The etymology of the term 'hippie' is from hipster, and was initially used to describe beatniks who had moved into San Francisco's...

culture.