If the dog's gastrointestinal bleeding occurs, the dog's feces will turn black. At the same time, the dog will be depressed and have a loss of appetite. At this time, the dog needs to be injected with a hemostatic injection.
Pet News
What causes dogs to have black poop?

This condition in dogs may be caused by eating too much black food, or bleeding from the stomach or foregut. When a dog eats too much black food, the dog’s feces will turn black due to food pigment problems. This is normal and owners do not need to worry too much.