Hi all,
I have 2 varieties of grapevines: a Muscatel variety from which I harvested over 100kg last year and turned it into wine. This year I have the same problem as RiumarPat in that the fruit started off with a blue/grey tint and then matured to brown empty skins. I managed to collect about 10kg of useable fruit. The other vine has small black grapes that make excellent wine or for juicing. These have not been affected at all.
I have done some research and discovered that the problem is "powdery mildew". Here is a link to a useful article and how to prevent it:
http://organicgardening.about.com/od/diseases/p/PowderyMildew.htm
Good luck,
Simon