Preheat oven to 350 degrees F. For topping, in a small bowl, combine brown sugar, nuts, and cinnamon. Set aside. In another small bowl, stir together sour cream and baking soda. Set aside. Grease a 13x9x2-inch baking pan. Set aside.
In a large bowl, combine granulated sugar and softened butter; beat with an electric mixer on medium speed until well mixed. Beat in eggs and vanilla until well mixed. Add the flour and baking powder; beat until well mixed. Add the foamy sour cream mixture; beat until well mixed.
Spread half of the batter into the prepared pan. Sprinkle blueberries over batter. Sprinkle half of the topping over the blueberries. Carefully spread remaining batter over the topping; sprinkle remaining topping over the batter.
Bake in the preheated oven for 35 to 40 minutes or until a toothpick inserted near the center comes out clean. Drizzle with Powdered Sugar Icing. Serve warm, or cool on a wire rack.
In a small bowl combine powdered sugar, milk, and vanilla. Stir in enough additional milk, 1 teaspoon at a time, to make icing of drizzling consistency.
