Understanding Melena: What Does It Mean When You Have Dark Stools? Fortis Healthcare Black, Green or Bloody: What's Up With My Poop? Diet vs Disease 16 Ways to Achieve a Healthy Poop!! DrJockers.com What Causes Black or Tarry Stool? Dark stool vs black stool the difference can be serious. Dark brown stool is often food or iron related. Black, tarry stool may indicate internal bleeding and needs urgent medical attention.