Dogs Skin Turned Black

So we bought our House Last Sept.  And ever since they have been spending more time outside, their skin has gone from pink to pretty much black...and its happened to all of them...The Vet says they are healthy and theres nothing wrong with their skin.  Its just strange, I thought maybe it was from eating sticks and pine cones...but who knows.  Anyone else experience this before....Oh and I found this great BULLY PUP...not from a breeder but from a guy who was given 2 imported Bullies from his friends Dad that is a vet, because they were just too much for him. He just wanted to breed them once, but the pups and the parents are great...goin back to see em this weekend.  But what do yall think is up with this black skin