For the coffee cake:
1. Sift the flour, baking soda, baking powder and salt together.
2. Place the butter in the bowl of an electric mixer and beat on high speed until the butter is soft. Add the brown sugar and continue beating until the mixture is light and fluffy.
3. With the mixer on low speed, add the eggs one at a time and then add the vanilla.
4. Using a rubber spatula, fold in half of the flour mixture, then the sour cream and applesauce and finally the remaining flour mixture.
5. Stir in half of the blueberries.
6. Spoon half of the batter into a 9 or 10-inch bundt pan. Sprinkle the batter with half of the streusel topping and the remaining blueberries. Spoon the remaining batter into the pan and sprinkle with the remaining streusel topping.
7. Place the pan on the center rack and bake for 40 to 45 minutes or until a knife inserted into the cake comes out clean. Place the pan on a wire rack and cool for 10 minutes before removing the cake from the pan.