Line a 9 inch pie pan with half the pastry. Save remaining pastry for top crust. Chill both while preparing the blackberries.
Combine berries, flour, sugar, and lemon juice. Spoon into pie shell, and dot with butter or margarine. Cover with top crust, and slash in several places.
Bake for 15 minutes. Reduce heat to 350 degrees F (175 degrees C). Continue baking for 35 to 40 minutes, or until browned.
