You can tell a lot about your dog's intestinal health by evaluating his stool. It's a quick and simple method.
The stools of a healthy dog are formed and easy to scoop. Any color is allowed as it depend on the kind of dog food you're feeding, though black stools are not normal. When your dog's stool is black this can be the result of gastrointestinal bleeding and an indication to make an appointment with your vet.
An occasional loose stool is normal. Perhaps your dog was a bit tense which could also result in loose stool. Nothing to worry about. When this continues and you see loose stools days after day then schedule a veterinary visit. When your dog has very watery diarrhea then call your vet right away.