Blood in the gastrointestinal tract chemical reaction color will be black, with the stool out of the body will appear black feces, may be eating blood products such as food and other dietary factors, but also may be gastric ulcers and other diseases lead to gastrointestinal bleeding, or taking aspirin and other drugs caused by gastrointestinal bleeding. 1. Diet: may be eating duck blood and other blood products, blood in the digestive juices and other substances under the action of the chemical reaction, the color becomes black, bowel movements appear black. 2. Gastrointestinal diseases: gastric ulcer, gastritis and other gastric mucous membrane bleeding, esophageal – fundal variceal rupture bleeding and other upper gastrointestinal bleeding, red blood in the gastrointestinal tract after the reaction of the discharge of the body, will also appear in the stool black. 3. Drugs: taking aspirin and other non-steroidal anti-inflammatory drugs, warfarin, rivaroxaban and other anticoagulants, inhibit platelet aggregation, easy to cause gastrointestinal bleeding, there will also be black stool. The appearance of black stool to exclude eating and other physiological reasons, may also be other reasons, should be timely consultation, under the guidance of the doctor to identify the causes, and timely treatment, so as not to delay the best treatment period.