What causes black stools? can dehydration cause black stools?

The most serious thing to check is to be certain this is not bleeding. Black stools can represent bleeding higher in the GI tract. The stools should be checked for microscopic blood and, if positive, a GI doctor should be seen. An EGD and colonoscopy will then likely be needed to find out exactly where there is bleeding and what the diagnosis is-that is, what the etiology of the bleeding is.

It is the type of food ingested, sometimes drugs or digested blood that can cause black stools, dehydration is not likely to cause the change of color, it can increase consistency.