My Dog Has Black Spots On His Skin How Come My Rat Terrier Has Gotten Black Spots On His Skin?

How come my rat terrier has gotten black spots on his skin? - my dog has black spots on his skin

I know that has rat terrier with black spots on the skin, but my dog developed over a period of several months. His nose has become dark and uses it to stop, pink, and it is now darker. He has five years and is healthy and active. This is not the place of the earth, if not eliminated. I also saw this happen with other dogs. What's the deal? Please only serious answers.