Black diarrhea can signal bleeding from the intestines or the stomach. It may also be a reaction from certain medications, such as Pepto-Bismol, according to Williams College.
Eating too much black licorice, taking iron supplements or eating blueberries can cause loose, black stools, according to MedlinePlus. Other causes of black diarrhea include a bleeding stomach ulcer, gastritis, which is an inflammatory condition of the lining of stomach, an esophageal tear from protracted vomiting and widened veins in the stomach or esophagus. Tests that are performed to determine the pathology of black diarrhea include barium studies, a colonoscopy, a complete blood count and stool cultures.