High blood sugar does not usually lead to black stools. Black stools may be caused by improper diet, or by diseases such as esophagogastric fundus varices and gastric ulcers. 1. Improper diet: If you have recently eaten too much animal blood products, kelp and other iron-rich foods, you may change the color of your stools, resulting in black stools. 2. Esophagogastric fundal varices: rupture of esophagogastric fundal varices may cause hemorrhage, so the patient may have black stools, vomiting blood and other symptoms. 3. Gastric ulcer: The causative factor of this disease is Helicobacter pylori infection, followed by non-steroidal anti-inflammatory drugs. Common risk factors for gastric ulcers are smoking, drinking alcohol, strong tea, and excessive stress. Some patients may experience dark stools, abdominal pain, and heartburn. When patients with high blood glucose experience dark stools, they should visit the hospital and have relevant tests to clarify the exact cause.