Place an oven rack in the bottom third of the oven. Heat the oven to 400º F. Line a baking sheet with aluminum foil or parchment paper and set aside.
If using homemade pie crust, roll out the pie dough into two 12" circles. If you're using store-bought crust, let it sit at room temperature for a few minutes to soften slightly. Line a 9" pie dish with one of the pie crusts. Place the lined pie dish and the top crust in the fridge to chill while you make the blackberry filling. 2 9" pie crusts
Place the frozen blackberries in a large microwave safe bowl. Microwave the berries for about 30 seconds. You want to microwave them just enough so they become a little juicy, but not so much that they completely defrost.
4 cups frozen blackberries
Add the sugar, lemon juice, salt, and cornstarch to the blackberries. Stir together until the berries are coated and no dry cornstarch remains.
¾ cup sugar, 1 TBS lemon juice, ¼ tsp salt, 5 TBS cornstarch
Pour the berry filling into the lined pie dish. Use a spatula to spread it out evenly. Evenly distribute the cubed butter over the top of the berry filling.
2 TBS unsalted butter, cubed
Add the top crust. For the top crust you can either make a lattice top or simply roll the top pie dough round over the berries and cut a few slits to allow moisture to escape during cooking. Trim, seal, and crimp the edge of the pie.
Whisk an egg in a bowl. Using a pastry brush, carefully brush the egg wash over the top of the pie. Sprinkle the coarse sugar over the top of the pie.
1 egg, 1 tsp coarse sugar
Place the assembled pie on the lined baking sheet (the baking sheet catches drips during baking). Put the pie in the oven and bake for 45 - 50 minutes. After 25 minutes, cover the edges of the pie with foil to prevent over browning.
Remove the baked pie from the oven. Let it cool on a wire rack for at least 4 hours to completely cool and set. Once completely cool, slice and enjoy!