the term for that condition is mellanistic, a mutated gene which makes the hair on part or all of the body black, the odds of a deer carrying this gene has been stated to be as high as 4,000,000 to 1. much rarer than piebalds or albino's
here is a pic of one taken in tx several years ago
mellanism is pretty common in other animals, I've taken over a dozen mellanistic fox squirrels here in wv. on most of them the hair on them which should be white is black