What causes a dogs poo to be black?

The only thing I would think of is old blood or something that he ate. Usually blood in poop has a really thick tar like consistency. I would keep an eye out for he next couple of stools and make sure they were not all that way. If they are I would say take your dog to the vet. However if it is a one time thing it may just be something the dog ate.

Seems to be after she eats raw hide chews. Hm?

I would stop giving them to her. I was told that some brands of rawhide chews contained plastic. To be on the safe side I would stop giving them to her and watch her poo carefully.

Forget the rawhides. They kill, plastic or no plastic.

The pet stores are full of toys that many dogs will quickly chew up into
Pieces they could choke on or cause intestinal blockages. If you are not
There to watch, stick to sturdy stuff such as Nylabones and Kongs. Keep a
Close eye on chew toys and quickly discard anything that is coming apart in
Pieces. Rawhide is especially bad because it swells after being swallowed.
These problems are the worst with, but not limited to, large, aggressive
Chewers such as Labs.

I agree to stop feeding rawhides, It could be blood in stool, but you also should go to the vet, check out how much iron is in your dogs food or supplements/treats as too much iron in humans causes black tarry stool, not sure about dogs.

I guess I got lost on rawhides. The dog still needs to go to the vet.