Actually, I'm not sure what kind of grapes they were--they were red, tho. I live in England, and we don't have concords here, more's the pity. My brother-in-law grows these, they were already in his greenhouse when he bought the house, so we really don't know what kind they are! I used straight juice, tho, not half and half water. NT suggests to drink it half water and half cooler, after the cooler is done.
Sounds as if your cooler is actually okay, tho. I, too have the Maker's Diet, but didn't think to look at that recipe.