Chinese five-spice powder and ground pepper give this down-home pie a hint of sophistication, but the filling will also be delicious if you leave them out.
Preheat oven to 375 degrees. Make the crust: Unfold pastry on a very lightly floured baking sheet, and roll out to a 1/8-inch thickness. Set a 9-inch pie plate, upside down (do not press down), on 1 side of pastry, and cut around plate using a sharp paring knife. Remove dish. Cut five 3/4-inch-wide strips from scraps.
Brush round with egg wash (do not let drip down sides), adhere strips to surface, and brush again with egg wash. Cut out a 2 1/2-inch hole in the center of pastry round using a biscuit cutter. Sprinkle with sanding sugar. Freeze for 15 minutes.
Make the filling: Mix together plums, blackberries, granulated sugar, cornstarch, five-spice powder, 1/4 teaspoon salt, and 1/8 teaspoon pepper.
Pour filling into pie plate, and cover with parchment-lined foil. Transfer to a foil-lined rimmed baking sheet.
Bake pastry crust on top rack and filling in bottom third of oven for 30 minutes. Transfer crust on baking sheet to a wire rack (it should be golden and cooked through). Let cool slightly.
Continue to bake filling until starting to bubble in center, about 30 minutes more. Remove from oven, uncover, and top with pastry crust.
Bake pie until bubbling vigorously in center, about 10 minutes more. Transfer pie to a wire rack, and let cool for at least 1 hour before serving.