I cannot really tell anything from the photo you posted.
Has your dog been licking or chewing at the area?
Is it just discoloration, or more like a wound?
My dog suffers from allergies and he will lick or scratch at a place till it's raw and the fur is gone, if I do not keep him on allergy medicine at this time of the year. When he does that the area will end up darker then the rest of the skin but it's also reddish rather then a dark bruise color. The other thing about him is that his skin naturally has dark spots, and I've had to look twice to make sure what I was seeing was just one of his dark patches and not a bruise.