A black deer is even more rare than an albino, it is called melanistic (not sure on the spelling) that black buck in the picture i know was killed in texas, i believe there was an article about it on the north american whitetail website.
this about explains it, its no different the a regualr spotted leopard, and a black leopard(most comingly refered to MISTAKENLY as a black panther)
while an albino is missing its skin/fur pigments, a malonistic creature has "full" pigmants meaning it doent have color tones its just black. although as seen on the leapords, you will still be able to see the "rosettes" or spots. my uncles tiger(he is the one that caused me to get big cats) once had a malonistic cub.