I think the red in grape juice can effect stool color (I think more to the greenish..but could be green black).As long as the black is not tarry.If you have gastric irritation the alcohol might be enough to cause bleeding...maybe????But it may just be a color from the red in the grapes.It might be worth seeing if white wine does the same thing. That would narrow it to the color from the grapes, rather than the alcohol.K.