In mixing bowl, beat together cake mix, sour cream, oil, milk and eggs on low speed for 30 seconds. Increase speed to medium-high and beat for 2 minutes. Pour batter into prepared baking pan.
Toss blueberries with one tablespoon of flour and sprinkle blueberries on top of batter.
Combine brown sugar, granulated sugar, 1/3 cup flour and butter in a bowl. Work together with your fingers until mixture just starts to stick together. Sprinkle over blueberries.
Bake for 35-45 minutes or until tester comes out clean. Remove from oven and cool for 30 minutes.
To make the glaze, place powdered sugar in a bowl. Add one tablespoon of milk and stir. Add remaining milk in small amounts until glaze is of drizzling consistency. Drizzle glaze over cake.
