Looks like that buck may have some melanistic traits in his gene pool.. I've seen pictures of bucks darker than that. But never alive pictures.
Melanistic is a form of genetic mutation in the color pigments.
Their are the normal color paterns that we see on deer every day but than their are Piebalds, Albinos, and Melanistic.
Piebalds tend to have both brown and white hair. They can some times be pure white. but their nose, eyes and skin tend to be the color of a normal deer.
Albino's have solid white hair with pink eye's, nose, and skin. The key is white and pink. If their is any color on the deer other than pink and white and dirt than it is not a true albino.
Melanistic deer are realy realy dark in color and can be black! Their almost the complete oposite of an albino.