Black blood in the stool is called black stool and is an indication of gastrointestinal bleeding. The blood stays in the gastrointestinal tract and is discharged through the anus with a black color, also known as tarry stool. The causes of black stool are: 1, rupture of esophagogastric fundic varices in cirrhosis, which is the most common cause of gastrointestinal bleeding. The patient has a history of cirrhosis, which increases the pressure of blood vessels in the fundus of the stomach and makes it easy to rupture and bleed. 2, cancerous bleeding: when suffering from malignant tumors of the gastrointestinal tract, black stool will be caused by the friction and rupture of the cancer. 3, bleeding caused by drugs: common are long-term oral aspirin, clopidogrel and other anti-platelet drugs, which cause black stool. 4, other diseases of the digestive tract Diseases: such as erosive gastritis, ulcers, enteritis and other diseases may cause black stools.