Causes Of Melena. Black stool or melena causes include: 1  Peptic ulcer that includes stomach ulcer, duodenal ulcer. 2  Stomach cancer. 3  Patient on anti-coagulant drugs such as Warfarin may cause melena. 4  Gastritis (inflammation of stomach) and esophageal varices.5  Hemorrhagic blood disease such as purpura.ymptoms of melena or blood in stool are: 1  Patient complains change of stool color to his medical attendant; it is dark black like coal tar. 2  Anemia is most important sign noted in patients with melena.