Welcome to Just Answer and thanks for your question. When there is bleeding in the upper part of the digestive system such as the stomach, duodenum, upper part of the small intestine, the blood turns black as it is acted upon by the digestive enzymes as it progresses through the system. That is why your doctor asks if the stool is black. If the bleeding should be from an internal or external hemorrhoid, the stool would be red. Other things that cause color changes in stool are things you eat or drink such as beets, licorice, Kool Aid, Jello, green vegetables, Pepto Bismol, iron tablets, etc. If there is a flare-up of diverticulitis, there might be blood, so your doctor is wanting to check with you to make sure.