Preheat oven to 425°F (220°C).
In a large mixing bowl, toss blueberries with sugar, cornstarch, lemon juice, salt, and cinnamon until well combined.
Place one pie crust into a 9-inch pie pan. Pour the blueberry mixture into the crust and dot with butter.
Top with the second pie crust. Trim and crimp the edges to seal. Cut slits in the top crust to allow steam to escape.
Brush the top crust with the beaten egg for a golden finish.
Bake for 45–50 minutes, or until the crust is golden brown and the filling is bubbling.
Allow pie to cool for at least 2 hours before serving to help the filling set.