Possible causes of black stools in stomach pain include improper diet and upper gastrointestinal bleeding. 1. Improper diet: eating a lot of pig’s blood and other iron-rich foods for a long time, or accompanied by indigestion, the phenomenon of black stool in stomach pain may occur. This is because unabsorbed iron ions are converted into ferrous sulfide in the intestines, making the stool black. 2. Upper gastrointestinal bleeding: peptic ulcer is the most common cause of upper gastrointestinal bleeding, patients have typical epigastric pain due to gastric mucous membrane irritation, etc. The ulcer erodes the surrounding blood vessels leading to bleeding, and the oxidation of iron ions in the blood makes the stools black. Gastric cancer, rupture of esophagogastric fundus varices and other causes can also lead to upper gastrointestinal bleeding. It is recommended that patients seek timely medical attention to clarify the cause of the disease for treatment to avoid serious consequences.