Heat oven to 400°F (200°C).
Place one crust into a 9-inch pie pan.
In a bowl, mix blackberries, sugar, cornstarch, lemon juice, and cinnamon until coated.
Pour filling into pie crust and dot with butter.
Roll out second crust, cut into strips, and weave into a lattice top.
Seal edges and brush with beaten egg.
Bake 20 minutes at 400°F, then reduce to 350°F (175°C) and bake an additional 30–35 minutes, until filling is bubbling and crust is golden.
Cool at least 2 hours to set. Serve plain, or with ice cream or whipped cream.