Stir cornstarch into creme de cassis and lemon juice until smooth. Set aside.
Combine 1 cup of the blueberries with Wild Maine Blueberry Jam in saucepan and bring to a boil over medium-high heat. Cook until preserves are melted and berries start to pop. Stir in the remaining 1 1/2 cups of berries and peaches. Pour in lemon juice mixture and continue cooking for 2 minutes longer. Fruit mixture will be quite thick. Set aside to cool, about 8 minutes.
Line a 9-inch pie plate with pastry, leaving overhanging edge untrimmed. Brush with egg white. Pour cooled fruit mixture into pie shell. Roll out remaining dough into a circle and cut into 1/2 inch strips. Arrange them over pie in a lattice pattern and trim. Bring edge of bottom crust over rim and crimp edges.
Bake pie on a cookie sheet in the middle of oven until crust is light brown, about 8 minutes. Adjust heat to 350 degrees F and continue baking until crust is golden brown and juices begin to bubble, 25-30 minutes. If crust becomes too brown before end of cooking time, cover with foil. Remove pie from oven and let cool for at least 30 minutes before serving. Serve with vanilla or peach ice cream, if desired.