We tried this with the mass amounts of blackberries we have in our house. We wanted jam without dealing with fruit pectin, and this turned out way too sweet. I would recommend using more lemon juice/less sugar and also mashing the berries beforehand.
Yesterday The Girl and I picked blackberries. After after gorging ourselves on fruit, we made a cobbler. But we still had a huge bowl of blackberries left over. So I popped them in the freezer last night and made a batch of blackberry jam this.