A black stool in a very sick elderly person should be due to gastrointestinal bleeding in the elderly. Gastrointestinal bleeding is divided into upper gastrointestinal bleeding and lower gastrointestinal bleeding. The common causes of upper gastrointestinal bleeding are gastric ulcer, duodenal ulcer, gastric cancer, etc. These causes cause the rupture of small blood vessels under the mucous membrane, and the blood component, under the action of gastric acid, becomes coffee-like or black material, and the coffee-like material can be discharged by vomiting, while the coffee-like material can be discharged by feces and become black stool. The most common cases of lower gastrointestinal bleeding are tumors of the colon or rectum, ulcerative colitis, etc. The tumors invade the blood vessels under the mucosa, and the blood vessels rupture and bleed, and the blood is black under the action of digestive juices, and is discharged with the stool, so you can see the elderly having black stools. There is also a case that the elderly have other diseases, because of stress ulcers caused by bleeding in the digestive tract, this situation can also be seen in the stool is black.