I should add that it has rained significantly more than usual since the first week of July.

Powdery mildew. Get a spray bottle and mix half milk and half water. You may have to spray a couple times. I've only had to spray once to knock it out. The leafs won't repair them selfs but it will kill the mildew. Rabbiteyes are bad about leaf fungus. I like to wait until after harvest and do a summer prune. Let the cuts heal over good and then spray. It reduces the amount that has to be sprayed and let's you get better coverage because there's less to deal with. Powdery mildew won't kill your plant but it is unsightly and will cause early leaf drop on Rabbiteyes. Another benefit of summer pruning is it forces new wood that will retain leafs longer into the fall so there's better bud set.

poolecw,

You have black spot fungus not powdery mildew. There are a lot of natural alternatives to stop the spreading of this fungus. Try a google search.

Ron

It looks like a fungus to me. It really is no big deal. I tend to ignore it.

I have it on most of my blueberries too.