Unfortunately the photos are too small for us to diagnose, but common issues with grape fruit development stem from a Black Rot infection, a fungal disease. Information about this disease and prevention techniques can be found in our Growing Grapes in a Home Garden web page. (See the "plant and pest problems" section.) You're welcome to resend images if you aren't sure if the symptoms match any of the issues discussed in that page. (Please use images at least 1MB in file size so they are large enough for us to enlarge.)

Miri