Heat oven to 325 degrees F. Liberally butter a 9 x9 square metal or ceramic cake pan.
In a small bowl, combine sour cream and baking soda. Set aside to react. It might not look like much, but when you break the surface you'll see lots of little bubbles.
Beat the butter in a mixer until light and fluffy. Add sugar and beat until well combined. Add eggs one at a time, beating between each addition, then add vanilla extract. Add baking powder. Add one cup of flour, then the sour cream mixture, then the remaining flour, beating just until combined.
In a separate bowl, mix together the topping ingredients.
Spread half of the batter into the prepared pan. Sprinkle the blueberries evenly over this base layer, followed by 1/3 of the topping. Spread the other half of the batter into the pan, taking care not to smear the filling. Sprinkle the remaining topping over the cake. Bake for 60 minutes or until a toothpick inserted comes out clean. Serve warm if possible. Reheat as necessary, covered, in a 200-250 degree oven.
*If you buy frozen, be sure they are not sweetened. Do not thaw. You could also use raspberries or blackberries. Mmm.