The human eye has an ostensible perceptive wavelength range between 380–780 nanometers. Within those limits, imagery produced via the eye is practically perfect; an unfathomable physiological feat. Of course, what we call visible light is a narrow band in the electromagnetic spectrum.
A black light is a lamp that emits relatively long-wave ultraviolet (UV) light, just outside the boundary of human vision. Many substances fluoresce: reflect light at a different wavelength, typically longer than that absorbed. Greeneye fish and other deep-sea animals use florescence to see.
While the UV of a black light cannot be seen by humans, reflected black light is visible, and so useful for spotting the otherwise unseeable. Applications include forensic finding of fingerprints and blood, dermatological detection of skin conditions, various non-destructive tracking of biological molecules, and for gazing at trippy psychedelic posters.
By contrast to human vision range, reptiles can see in the infrared spectrum, while spiders and many insects, including bees, see ultraviolet light.
Plants evolved their flowers to please their pollinators, with accent and guides that let insects readily collect their reward for providing plant pollination service. Colors are particularly striking in the yellow spectrum which butterflies are especially sensitive to, and in the ultraviolet spectrum, where bees have fine visual discernment. Flowers that look a consistent yellow to humans have a much more striking appearance to butterflies and bees, which can perceive subtler gradations. A dash of ultraviolet gives a bee good guidance.