If memory serves me correct, that's called Blue Rot and is a fungus. At the point your pictures were taken, you probably won't be able to salvage this year's crop. Next spring just prior to the buds opening up, you'll need to spray with a fungiside. I don't remember any particular names, but it will tell you on the label what fungus's it will work on. You'll need to spray at least every couple of weeks as the grapes mature. Probably could stop as they start ripening. Read the directions for frequency of spraying or if you have a Farmer's Co-op, they should be able to give some help.
