I think it has something to do with the intensity setting of each light...if you look at the intensity dial, it is nearly off or off on the black lights. If you move it to full, the light becomes white.... If you change the light color to, red, as an example, then turn the intensity down, it will become black.... So basically, when you have black lights, the intensity is very low

Humankind has not woven the web of life. We are but one thread within it.
Whatever we do to the web, we do to ourselves. All things are bound together.
All things connect......Chief Seattle, 1854