A "blackberry summer" is cold and wet. My summer started out that way. We were still using space heaters in April and our area experienced massive flooding in May.
I can see why the blackberrys would respond...the rest of my garden is blooming and growing more than ever.
Things have dried out and heated up, just as expected. The oppressive heat will continue through August with little relief.
The climate hasn't done much for my sunflowers, though.