Bleeding that takes place in the esophagus, stomach, or the first part of the small intestine most often causes the stool to appear black or tarry. Maroon-colored stools or bright red blood often mean that the blood is coming from the small or large bowel, rectum, or anus.
https://medlineplus.gov/ency/article/003130.htm
Melena: Stool or vomit that is stained black by blood pigment or dark blood products. Last Editorial Review: 5/13/2016.
http://www.medicinenet.com/script/main/art.asp?articlekey=4343
Other names or spellings for Melena: PR bleeding, Gastrointestinal bleeding, Anal haemorrhage, BRBPR, Bleeding per rectum, Blood in faeces, Blood loss from GI tract, Bloody stool, GI bleeding, Gastrointestinal haemorrhage, Gastrointestinal hemorrhage, Hematochezia, Melaena Source - Diseases Database.
http://www.rightdiagnosis.com/m/melena/basics.htm
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http://symptoms.rightdiagnosis.com/cosymptoms/melena.htm
Bleeding in the esophagus or stomach (such as with peptic ulcer disease) can also cause you to vomit blood. Bleeding that takes place in the esophagus, stomach, or the first part of the small intestine most often causes the stool to appear black or tarry. Your doctor may use the term melena.. Bleeding in the upper part of the GI tract will most often cause black stools due to: Abnormal blood vessels.
https://medlineplus.gov/ency/article/003130.htm
In these situations, the blood is moving through the colon so rapidly that there is not enough time for the bacteria to turn the blood black. Sometimes, bleeding from the gastrointestinal tract can be too slow to cause either rectal bleeding or melena. In these patients, bleeding is occult (not visible to the naked eye). The blood is found only by testing the stool for blood (fecal occult blood testing) in the laboratory.
http://www.medicinenet.com/rectal_bleeding/symptoms.htm
In medicine, melena or melæna refers to the black tarry feces that are associated with upper gastrointestinal bleeding. The black color is caused by the hemoglobin in the blood being altered by digestive chemicals and intestinal bacteria.
https://en.wikipedia.org/wiki/Melena
Diagnosis[edit] In acute cases, with a large amount of blood loss, patients may present with anemia or low blood pressure. However, aside from the melena itself, many patients may present with few symptoms. Often, the first approach is to use endoscopy to look for obvious signs of a bleed.
https://en.wikipedia.org/wiki/Melena
Causes[edit] The most common cause of melena is peptic ulcer disease. Any other cause of bleeding from the upper gastrointestinal tract or even the ascending colon, can also cause melena. Melena may also be a sign of drug overdose if a patient is taking anticoagulants, such as warfarin.
https://en.wikipedia.org/wiki/Melena
In some patients, bleeding can be black and tarry (sticky) and foul smelling. The black, smelly, and tarry stool is called melena. Melena occurs when the blood is in the colon long enough for the bacteria in the colon to break it down into chemicals (hematin) that are black.
http://www.medicinenet.com/rectal_bleeding/symptoms.htm
what is melena diagnosis
Melena diagnosis is the black tarry feces that are associated with upper gastrointestinal bleeding. Melena  refers to the black tarry feces that are associated with upper gastrointestinal bleeding. The diagnosis for melena is black tarry feces that are associated with upper gastrointestinal bleeding.