I've a fawn 5 months old boxer puppy. A few days ago I noticed that his scrotum had become a little bit black. It have been pretty cold here recently (-15 celsius) and I was wondering if he might have gotten a frostbite on his scrotum? The area is not cold but black but warm, he don't seems to care about it at all. Should I take him to vet to check or is it just pigment?