what causes this? how to get rid of/treat it? doesn't seem to be affecting the fruit blossums.
It will affect fruit later Florida is land of downy mildew and south Georgia. Florida has some best info on Downy Mildew around the below some good reading.
Blackberries are most affected by Mildred than any other disorder.
Here is a link that might be useful: Link showing early stage Downy Mildew
Can you spray a fungicide?
already have some berries so don't want to spray any chemicals. don't want to spray chemicals regardless. looking for an organic answer if possible.
Joe, the two most common diseases doing what you are seeing are septoria leaf spot and blackberry rust. My guess would be the former since it is more common. My recollection is dormant lime-sulphur is the best treatment. Lime-sulphur is organic. Since you may be past dormant season you could still spray a less potent strength I expect but look up some guidelines for treatment of the disease with Google to get the details. But, before you do anything search for pictures of various blackberry leaf diseases and figure out which one you have for certain. Then search for treatments for that. If you are not sure, ask more questions here :-)