if she ate spinach or something like that at her gran's house, you probably have the answer, but if not, then it would be worth your doctor checking a sample for blood. Poos can turn black if the gut is digesting blood, which could be a sign of internal bleeding, or for example, swallowed blood from a nosebleed.However if she is otherwise ok, then internal bleeding wouldn't be excessive.
Don't want to worry you unduly; why not ring her grandma and ask what she ate?