My dog's skin is turning black... help!

I have a 7-year-old chihuahua mix who has always been in perfect health. He eats human-grade high quality dog food and still acts like a puppy. Recently, I noticed that the skin around his anus had gone from being pink to being closer to black in color (I don't get too close to it, but it looks black from a distance). Last night, I noticed that the skin on his belly is also turning from pink to black. The only thing that is different in his routine is that he has begun taking daily walks. Has anyone ever heard of this sort of thing? I don't know if I should take him to the vet or if they'd laugh at me and say I'm just a hypochondriac! I just know that his skin has never been dark or black before. Tweet