In large bowl, combine cornstarch with remaining 1/2 cup sugar, making sure to break up any clumps. Add berries and toss around to coat. Spoon the berries over the crust and sprinkle with remaining crumb mixture and juice. Cover edges of pan and bars with strips of foil to keep from burning.
Note; if you use sweet blackberries, add 1 teaspoon of lemon juice to the berries to keep it from being overly sweet.