Since blackberries are firmer and bigger than blueberries, I crushed the berries with the sugar, lemon juice and cornstarch before adding to the pan. I think it helped break them down a bit. The only other modification I made was to reduce the sugar (only slightly), by about 1/3 cup. I just remember them being so sweet that a little less sugar couldn't hurt. It wasn't a noticeable difference, at least to my mouth.
I still ate them up. Next time, I'll try them with lime juice instead of lemon.