Preheat your oven to 400°F.
Combine the blackberries, sugar, cornstarch, lemon juice, lemon zest, and vanilla in a large mixing bowl. Gently toss to coat.
6 cups blackberries, 1/2 cup granulated sugar, 1/4 cup cornstarch, 1 tablespoon lemon juice, 1 teaspoon lemon zest, 1 teaspoon vanilla extract
Line the bottom and sides of a pie dish with one of the pie crusts, ensuring the crust comes up the sides of the dish evenly.
2 pie crusts
Pour the blackberries and any residual sugar and cornstarch from the bowl evenly into the pie crust.
Top with a second pie crust, pinching it together with the bottom crust.
Whisk together the egg with a tablespoon of water. Brush onto the crust just enough to coat it lightly.
1 egg
Bake the pie for 15 minutes, then lower the oven temperature to 350°F and bake an additional 45 minutes, or until the crust is golden brown.
Allow the pie to cool before slicing and serving.