My dog has black spots all over his private area

Hello  I am struggling to orientate myself on these pictures and might not immediately have recognised them as testes without your description - such is the nature of answering questions online.  Please therefore understant that I will talk about the blackening of testes in general.   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 inection (eg yeast, bacteria or mites), injury (eg following a dog-bite, which would also be infected) or cancer, so this is worth taking seriously.   I'm sure that this must be painful, whether or not obvious signs of pain are being seen, so would strongly advocate a vets' visit.    Your vet will look at this in context and may opt to give pain reflief and / or to take samples.