Preheat oven to 350 degrees. Generously butter and flour a 9-inch Bundt pan, or a 13 x 9-inch baking dish.
In a large bowl, using a mixer, cream together butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in sour cream and vanilla. In another bowl, combine flour, baking powder and salt and stir into batter just until blended. Fold in blueberries with spatula.
Spoon half of batter into prepared pan. In a small bowl, stir together brown sugar, cinnamon and walnuts. Sprinkle half of this mixture over batter in pan. Spoon remaining batter over top and then sprinkle remaining walnut mixture over top.
Bake in preheated oven for 55 to 65 minutes for Bundt pan, or 45 to 50 minutes for baking dish, or until a toothpick inserted into cake comes out clean. Cool in pan on a wire rack. Invert onto a serving dish, and tap firmly to remove from pan. Dust with powdered sugar just before serving. Makes 1 cake. Enjoy!