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Melanosis coli is dark brown pigmentation of the colon that occurs with the use of laxatives containing anthraquinone, such as senna. It can develop within a few months of use, and it can disappear in a few months if the use of the laxative is discontinued.
from my own experience, what you refer to as black colon is also called  melanosis coli which means: it's a condition associated with chronic laxative use in which dark pigment is deposited in the lining layers of the large intestine(which is the colon). the pigment results in a characteristic dark brown to black lining of the large intestine.