NANCY:
Good Afternoon. All you have to do is defrost the frozen blackberries & save the liquid as well. Then place all or about 90% of the berries in a food proccesser & puree this batch. Then place this concoction along with the berries that were not included in a sauce pan. Mix cornstarch with very cold water (about 2 tsp. cornstarch to 1/2 cup water) with a little sugar, mix & blend well. As the concoction is heating & beginning to simmer mix in the cornstarch solution & keep mixing till it coagulates (Thickens). Cool & enjoy.
I hope this helps you. Have a nice day young lady.
~Chef Dunask.