Tarry black stools are often a sign of bleeding in the upper digestive tract. Sometimes dogs can have allergic reactions to proteins--and it can cause inflammation and sloughing in the intestinal tract. So I think a visit to a vet would be a good idea, too, just to rule out certain things that could become serious problems in the future if left unaddressed.
I hope you can get the root of the problem soon, Bo and Spectre! Welcome to the board!
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."