In a mixing bowl, gently combine pitted cherries and blueberries with lemon juice, sugar, and cornstarch. Stir the mixture thoroughly for about 2 minutes until coated.

Roll out the chilled pie crust on a floured surface until 1/8 inch thick. Transfer it to a 9-inch pie dish and preheat your oven to 375°F (190°C).

Pour the fruit filling into the pie crust and dot the top with small pieces of butter. Roll out a second crust to cover the pie, crimp edges, and make slits in the top crust.

Bake the pie in the preheated oven for 45-50 minutes until the crust is golden brown and the filling is bubbling.

Remove the pie and let it cool on a wire rack for at least 2 hours before serving.