Black long hairs do exist... although they don't look jet black, because of the long hair. The under-fur looks greyish, as it does in Black Persian cats, because they have the same amount of pigment granules as a short hair but obviously stretched along a longer hair length.
Female black long-hairs would look blacker, as their coats are shorter!
This is Albert Einstein, my Black banded Long-hair:
and here's Forte, my Black Rex:
(a bit blacker now as he's had a good moult!)
I may have some Black babies early in the New Year.....