Preheat oven to 400 degrees. Arrange blackberries in pie shell. Mix remaining ingredients in a medium bowl until well combined. Pour over berries. Bake 10 minutes, reduce heat to 375 degrees and bake an additional 30 - 40 minutes or until a knife inserted in the middle comes out clean. Cool prior to slicing. Top with whipped cream, if desired. Store pie in the refrigerator.