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I provided as much of an answer as can be provided over the internet.
As I said the other day, the primary concern when there is black stool is that it may represent digested blood. When there is bleeding into the stomach or upper small intestine, then the blood is subjected to the digestive processes and will turn black. This can occur from many different conditions, such as ulcers or gastritis or abnormal growths in the stomach. There are other possible causes of black stool, including iron supplements, foods that are high in iron, and certain medicines, such as Pepto-Bismol.
Since the stool has progressed to black diarrhea, it is much more likely that it is due to blood, but to confirm that it is blood and to determine the exact cause of the bleeding, it would require seeing your doctor for a proper evaluation. It is impossible for me to determine the cause of bleeding over the internet.