So "black lights" use a coating to restrict higher wavelengths of light. We see light from around 400-700nanometers...the Violet Indigo Blue Green Yellow Orange Red...ROYGBIV colors of the rainbow spectrum...where Violet is very short and red is very long (relatively). Infra red is beyond our ability to see, but does a great job warming our french fries, Ultra violet is too short for us to see (but not for bees to see) except for the very longest wavelengths. Photosynthetic organisms, including plants and the photosynthesizing zooaplankton in our corals, typically respond best to light in the lower end of the spectrum, with several active peaks at different wavelengths. Actinics are usually between 420-450nm, which have lower energy than say "white, 10K mH bulbs, but is where the corals tend to need light for producing pigments that we think look cool. That said, the 10K white bulbs do allow for arguably more growth because of more energy, but less coloration.
ok, enough with the science...Black Light is even lower than the violet/indigo 420nm, around 370nm. Though they do look cool, and make our corals floresce, they probably don't help them grow much.
Recall...the colors that we see are those that are reflected by an object...a red shirt absorbs most colors except red, black absorbs, white reflects. Black "light" is pretty much the absence of color and white light is all colors. If you shine a purple light at a white shirt (which reflects everything) it will look purple. A red light would make it look red.
happy reefing!
the Professor...