One of the best methods is to mix 3/4 cup sugar with each quart of berries, then freeze. You may then take out, mix with your cornstarch or whatever thickener you use and put in your pieshell and bake. They may also be packed in a sugar syrup and frozen, or just washed, drained and frozen.

Blackberry jam is wonderful,too. I did not care for the freezer jam myself, it had a perfumy taste to it, so I like cooked jam made with pectin. Just follow the directions on the package of pectin, then water bath for 10 minutes. Also, there is a product called Clear Jel that is like a commercial thickener to be used in making canned pie fillings. If you would like instructions on the Clear Jel, let me know. If you can't find it locally, you may order Clear Jel online.