Melena

n. dark or blood-stained feces; black vomit (Medicine)

In medicine, melena or melaena refers to the black, "tarry" feces that are associated with gastrointestinal hemorrhage. The black color is caused by oxidation of the iron in hemoglobin during its passage through the ileum and colon.
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Melena /me·le·na/ the passage of dark stools stained with altered blood. me·le·na n. The passage of black tarlike stools containing blood that has [more]Melena - Community and Resources

Noun
1. abnormally dark tarry feces containing blood (usually from gastrointestinal bleeding)
(synonym) melaena
(hypernym) fecal matter, faecal matter, feces, faeces, BM, stool, ordure, dejection

n. melena, dark or blood-stained feces; black vomit (Medicine)