Black stool may be because of eating dark foods or drinks, such as dark beer or beets, or taking certain supplements, such as iron. It can also be a sign of bleeding in the upper gastrointestinal tract, including the stomach, esophagus, or small intestine. If your stool is black, it is important to talk to your doctor to determine the cause. They may order tests, such as a stool sample or endoscopy, to help diagnose the underlying problem.