Bev, I have blackberries in my yard also. For my pies, I use 1 cup of sugar and 5 Tbsp of tapioca. It firms the filling up nicely, and tastes great. No one guesses that their is tapioca in there! I also have a Blackberry Crisp recipe that is always a hit. Great when you grow your own berries.
I rinse the freshly picked berries, roll on paper towels and freeze them on a cookie sheet so they don't stick together.