Pre-heat the oven to 425 degrees. Press the bottom part of the crust into a greased 9” pie dish. In a large bowl combine 3 ½ cups of blackberries with the sugar and flour. Spoon the mixture into the pie shell. Spread the remaining ½ cup of blackberries on top of the sweetened berries. Cover with the top of the crust and crimp the edges to seal. Brush the top of the crust with milk and sprinkle with white sugar. With a knife or fork make small slits in the top of the pie crust.
Bake at 425 degrees for 15 minutes. After 15 minutes reduce the temperature of the oven to 375 degrees and bake for another 20 to 25 minutes.