Thanks for your query..
Yes the dark grape juice can be the cause .Is the juice fortified with Iron? But there are other reasons for black stool
like Iron supplements
, Black licorice , Blueberries ,bismuth subsalicylate
and pathological conditions like bleeding gastric or intestinal ulcers.Dont panic. Stop the grape juice for a week and see the difference.If the colour turns normal then its ok ..If not then take your daughter to doctor.
Update here how she stays.