Black faeces where the colour is caused by bleeding in the intestine.
Blood in the faeces in which dark, tarry masses are passed from the bowel. It is due to bleeding from the stomach or from the higher part of the bowel, the blood undergoing chemical changes under the action of the secretions, and being finally converted in large part into sulphide of iron. It is a serious symptom meriting medical investigation. It can indicate peptic ulcer or carcinoma of the stomach.
Black, tarry stools result from bleeding, typically in the upper digestive system. Digestive secretions darken the blood. While melaena is often due to a peptic ulcer, it could also suggest the presence of cancer.