Hematochezia vs. melena: What's the difference? UT MD Anderson What is the difference between tarry stools and black stools in terms of gastrointestinal (GI) bleeding? What Your Poop Color Reveals About Your Health GASTROINTESTINAL HEMORRHAGE (2): Melena. Melena, the passage of black, tarry stools, usually implies blood loss greater than 500 ml originating anywhere between the oropharynx and the right hemicolon. Melena occurs most often, Foods That Cause Dark Stools and When to Be Concerned GoodRx