Preheat your oven to 425°F (220°C).
In a large bowl, combine blackberries, sugar, flour, lemon zest, lemon juice, cinnamon, and salt.
Toss gently until the berries are well-coated.
Roll out one pie crust and fit it into a 9-inch pie plate, leaving an overhang around the edges.
Pour the berry mixture into the pie crust.
Dot the top with small pieces of butter.
Roll out the second pie crust and place it over the berries.
Trim the excess dough and crimp the edges to seal.
Cut a few slits in the top crust to allow steam to escape.
Brush the crust with beaten egg to give it a golden finish.
Place the pie on a baking sheet to catch any drips and bake for 45-50 minutes, or until the crust is golden and the filling is bubbling.
Allow the pie to cool on a wire rack for at least 2 hours before serving.