My dog has black spots all over his private area

Please read this answer -fuller and more accurate.   Hello - I am struggling to orientate myself against this picture.  e.g. I can see something that might be scrotal skin, but it is difficult to appreciate which way up the dog is and the anatomy is not particularly clear.  If that skin is broken, as I initial thought, then it must be painful; if simply black, then perhaps not.   Please understand that I will talk about the blackening of testes in general, without referring to the picture.   Hyperpigmentation (going black) of the scrotal skin can be caused by exposure to sunlight, which causes extra melanin production.  However, melanocytes can also be linked to infection (eg superficial yeast, bacteria or mites), injury (eg following a dog-bite, which would also be infected) or cancer, so this is worth taking seriously.  The area may be painful, particularly if licked and whether or not obvious signs of pain are being seen, so we would strongly advocate a vets' visit.    Your vet will look at this in context and may opt to give trial treatment or to take samples.