Why is My Poop Green?

5. When Black Stools Indicate a Serious Condition

Stools changing from a normal brown or greenish color to black can also be a sign of a serious health issue if no foods were ingested to cause the change. If an individual is experiencing black stools that are also tarry in appearance, this could be a sign of digestive issues such as bleeding ulcers. Bleeding from the esophagus or stomach manifests as black stools, whereas bleeding lower in the intestinal tract, from the colon or rectum, is more likely to cause the stools to be red or have visible blood. Most black stools are caused by diet, Pepto-Bismol, or iron supplements, but it's important to see a doctor if the symptoms last for more than a few days.