Gather all ingredients. Preheat the oven to 425 degrees F (220 degrees C).

Combine 3 1/2 cups of blackberries with 1/2 cup sugar and flour in a large bowl. Spoon mixture into the unbaked pie crust. Spread remaining 1/2 cup of blackberries on top of sweetened berries.

Cover with the top crust. Seal and crimp the edges, and cut vents in the top crust for steam to escape.

Brush the top crust with milk and sprinkle with remaining 1/4 cup sugar.

Bake pie in the preheated oven for 15 minutes. Reduce the temperature of the oven to 375 degrees F (190 degrees C) and bake until filling is bubbly and crust is golden brown, an additional 20 to 25 minutes. Cool on wire rack.

Serve and enjoy!