Gently toss berries and 1/4 cup sugar in a large bowl; cover and chill 8 hours. Drain.
Fit piecrust into a 9-inch pieplate according to package directions; fold edges under, and crimp. Prick bottom and sides of piecrust with a fork.
Bake at 450° for 7 to 9 minutes or until lightly browned.
Stir together cornstarch and remaining 1 cup sugar in a small saucepan; slowly whisk in 1 1/4 cups water and vanilla. Cook over medium heat, whisking constantly, 7 to 8 minutes or until mixture thickens.
Stir together raspberry gelatin and blue liquid food coloring in a small bowl; whisk into the warm cornstarch mixture.
Spoon blackberries into piecrust. Pour glaze evenly over berries, pressing down gently with a spoon to be sure all berries are coated. Chill 2 1/2 hours. Serve with whipped cream, if desired.
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