I have gallbladder sludge, but am wondering what causes stool to be black?

GI Bleeding: Upper GI (gastrointestinal) bleeding can cause black stool (melena). This can occur with or without abdominal pain, nausea or vomiting. Common causes of upper GI bleeding include peptic ulcers. Ingestion of dark pigmented veges (e.g. Collards), iron supplements and bismuth containing digestive aids e.g. Pepto bismol (bismuth subsalicylate) also can cause the stool to become black. Contact your doctor asap.