Diarrhea is bad enough in dogs, but black diarrhea is a special cause for concern. It's usually blood that makes the stool black. Small dogs and puppies are at the most risk of dying from dehydration caused by diarrhea. Any time a dog's stools change color or consistency is a cause for concern. Black diarrhea calls for an immediate call to the vet.
Eating Bloody Foods
If your dog or puppy has gotten into garbage or rotting fresh meat, or eaten carrion, blood sausage or black pudding, then the blood in the stools is from the food. The diarrhea means the food has gotten your dog sick and you should call the vet.
Your dog or puppy could have swallowed something she shouldn't have, and that object caused internal injuries and diarrhea. Your dog needs x-rays and possibly surgery in order to live.
Other signs will include vomiting, with flecks of blood in the vomit, and sometimes pale gums. The dog needs to see the vet within 24 hours.
Another reason that there could be blood in the stool is cancer of the digestive organs or a reaction to medicines given for cancer. This also needs the immediate attention of a vet.
Do not give any human medication for acid indigestion or upset stomach unless under the instruction of your veterinarian. Human medicines are generally too strong for dogs, especially small dogs and puppies.