Extend the ladder to full height, position it underneath the motherload of just-out-of-reach blackberries, and drop it down on top of the canes. It's rare that any canes actually break, and I don't worry much about it because the canes are biennial and the ones with berries are going to die at the end of season. The root of the plant is perennial, and will continue growing new canes over top of the dried, dead ones. In fact, climbing up on the hillside aided by ladder, I've never seen the ground underfoot; this particular hillside might be composed entirely of dead blackberry canes.
I had to put on a sweater, long pants, and sweat me to death :-(
Looks like it works on berries, no bloody arms :-)