So long as Gunner is well, it's probably down to his food (or scavenging pick-ups!)
Buddy's goes dark when he has tripe or liver (I think?)
If he is showing any signs of pain, feeling sick or generally unwell then I would get him checked up as black poo can be a sign of bleeding internally (but it is rare, so don't panic!)
Hope he's ok. Do let us know.