Sometimes, black stools signal a problem. But not in every case. Black stool could be from a diet high in iron, dark leafy green vegetables or blueberries. Stool can also turn black from medicines with bismuth or iron supplements. Even licorice!
Doctors ask you to report black stools. This is because stool color can be a sign of bleeding from the stomach or the intestines. Blood that trickles from an ulcer or an irritated stomach lining is first exposed to stomach acid. Then, its bathed in digestive juices as it passes along your digestive tract. These washes change the color of blood from red to black. Bleeding that happens close to the end of the digestive tract and does not have a long, slow trip through the digestive tract stays red when it mixes with the stool.
If you have a fairly large amount of blood mixed in your stool, the stool would be black. It would also be soft and quite sticky like tar. Also, stool with blood has more odor than usual.
Your doctor can do a simple test to tell you if the black color has come from blood. This is called a guaiac test.
If your doctor is worried about your stool, you may need to have your stomach examined with a camera. This is a procedure called endoscopy or esophagogastroduodenoscopy (EGD).