I can't really see the lesions very well, but I'm guessing its black rot. I also have seen this on some of my grapes, and thought it was anthracnose at first. If the berries turn completely brown, then shrivel up to black mummies, it is likely black rot. Be sure and remove and dispose all the mummies before winter (this may not help much if you have lots of wild grapes around you). There is nothing you can do this year other than remove infected berries, but you will probably need to start spraying them in the future.

Esentially agree with K-nut; it's black rot or spot or whatever and you are pretty much toast this year - although I never actually tried "remediation" so much as "prevention". With respect to the latter, I have had good success with fungicide (I have used Mancozeb but there are others) application EARLY, which means beginning when you see about 1/4" of new green growth, and every 10-14 days thereafter until the withdrawal period.