Stools that are black in color or have an oily sheen of tar are black stools. When bleeding occurs, the red blood cells in the blood break down into iron sulfide during the retention process and are excreted in the stool. As a result, stools mixed with iron sulfide appear black and have a tarry sheen. Black stools are mainly caused by gastrointestinal bleeding, which may include ruptured varices in the fundus, acute enteritis, severe gastric ulcers, detachment of the gastric mucosa, and tumors. In addition, patients may also suffer from blood diseases such as leukemia and hemophilia, which may also cause bleeding in the gastrointestinal tract. If you find any of the above characteristics in your stool, it is recommended that you go to the hospital as soon as possible for specialized treatment.