What can cause a bowel movement to look like coffee grounds?

A dark or black, tarry bowel movement (melena) often indicates upper gastrointestinal bleeding and is almost always due to blood from lesions above the small intestine. Upper GI hemorrhage may be slow, chronic, and occult or may be rapid and severe enough to produce hematemesis and hypovolemic shock. When gastrointestinal bleeding is slow enough to allow digestion by colonic bacteria, the blood assumes a darker appearance, becoming black and tarry and resembling coffee grounds