This happens to my grapes every year without fail. I never get any grapes. This year I threw some wood ash on it and it seems to have prevented the rust spots on the leaves, but the grapes are still affected.

Can anyone help with a natural solution that actually works? I have more grapes this year than any other and I'd hate to lose them all...

Do those grapes shrivel and become black after being affected?

Yes, before they ever ripen.

OK, so it is Black rot.

First thing to do is to remove all the shriveled grapes ("mummies") and dispose of them: don't put them in the compost heap, burn or bury them deep.
This Fall after the leaves have fallen rake them and destroy them as well. If any are left on the vine, rip them off and give them the same treatment.
Next year, after the plant has started growing again, you will need to treat it with a Ziram- or Mancozeb-based fungicide.

This should be enough to put Black rot under control, but remember if you live in an area where Black rot is endemic, you will have to do this every year (and if there are native vines like Fox and Roverbank grapes it most likely is).