In a large saucepan, combine the sugar, cornstarch, cinnamon if desired, salt and water until smooth; add the blueberries. Bring to a boil; cook and stir for 2 minutes or until thickened. Set aside to cool slightly.
Line a 9-in. pie plate with bottom crust; trim pastry even with edge. Gently fold the strawberries, raspberries, blackberries and lemon juice into the blueberry mixture. Pour into pastry; dot with butter. Roll out remaining pastry; make a lattice crust. Trim, seal and flute edges.
Bake at 400° for 10 minutes. Reduce heat to 350°; bake for 45-50 minutes or until the crust is golden brown and filling is bubbly. Cool on a wire rack. Store in the refrigerator. Yield: 8 servings.