btw, it is called "backlighting", as in the letters on the keycaps being lit from the back as opposed from the front.
Keys don't have to be black.

"Blacklight" is when you have a lamp that radiates just outside the visible spectrum. Some fluorescent material emits visible light when lit by "blacklight". I don't know of any such keyboard. Be aware that even though some of these wavelengths can not be seen and cause the iris to close, they can hurt the eyes. UV lamps often emit also a visible light as an indicator that it is lit.
I know there has been at least one group order of parts for a custom backlit keyboard, but I am not too interested in backlit keyboards so I don't know the specifics.