Pick over the berries removing any debris. Rinse in cool water and drain well. Place the berries in a large mixing bowl. In a small bowl whisk together the sugar, cornstarch, salt and cinnamon. Sprinkle the sugar mixture over the berries. Add the grated apple, lemon juice and zest to the berries. Gently toss with a spatula until blended. Set aside for about 30 minutes while preparing the crust.
Preheat the oven to 400°F. Place a baking stone or baking sheet in the oven to preheat.
Roll out ½ the pastry and fit into the bottom of a 9-inch deep-dish pie plate. Fill the crust with the blackberry mixture, then dot with the butter. Top the pie with the remaining crust by weaving a lattice design or cut slits to vent a solid crust. Brush the egg wash over the top pastry. Lightly sprinkle sugar on top.
Bake the pie on the preheated pan for 30 minutes at 400°F then reduce the temperature to 350°F and bake an additional 30 minutes. Tent the pie crust with foil for the last 20 minutes of baking to prevent over-browning. The pie is done when the blackberry filling is bubbling and thickened, and the crust golden brown.
Cool to room temperature, slice and serve. Top with ice cream or whipped cream, if desired. Store leftovers in the refrigerator.