Preheat the oven to 425 degrees F (220 degrees C). Place 1 pie crust in a deep-dish pie plate. Set aside second crust.
Gently mix black raspberries, sugar, cornstarch, and tapioca together in a large bowl until berries are well coated; pour into the prepared pie plate. Top with butter pieces. Add remaining pie crust on top; cut slits to vent.
Bake in the preheated oven for 15 minutes. Reduce oven temperature to 350 degrees F (175 degrees C) and continue baking until top is golden brown, about 45 minutes. Let cool before serving.