Black color of stool may be caused by dietary factors, medication factors, gastrointestinal bleeding and other reasons. 1. Dietary factors: excessive intake of green vegetables, animal blood products and other dark-colored foods may lead to the formation of black stool color, and the color of stool will return to normal after stopping the consumption of these foods. 2. Drugs: Taking activated charcoal, iron supplements, bismuth (bismuth potassium citrate) and other drugs to treat gastric problems can also lead to black stools. 3. Gastrointestinal bleeding: When the amount of bleeding in the gastrointestinal tract is small, the hemoglobin of the oozing blood reacts with the digestive juices for digestion and metabolism, and black or tarry thick and shiny black stools will be formed. If you have black stools due to gastrointestinal bleeding, you need to go to the hospital in time to get a clear diagnosis and treatment under the guidance of your doctor.