First of all you are not Water Fasting so let's not call it water fasting. You are consuming some vitamins and juices, basically you are on a juice fast.
Black stool? Depends on how black it is. If pitch black like black hair then it is blood. If it is dark brown then it is junk waste that is sitting in your intestines for a long time, a good thing if that is the case.
If you get light headed or faint then I would worry. If this happens go to ER and let them check a smear of your stool, they can tell instantly if your bleeding inside.