Black stools, in dogs and in humans, can be an indicator of internal bleeding, and needs to be checked out. Call your vet about this. Collect a small sample of the black stool, put it in a clean sandwich bag, and take it to the vet so they can look at it under the microscope to see what's going on.
A black stool caused by food, supplements, medication, or minerals is known as "false melena." It is called "false" because the black color is not due to blood. Iron supplements, taken alone or as part of a multivitamin for iron-deficiency anemia, may cause black stools or even green stools. Foods that are dark blue, black, or green in color may also cause black stools.