In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla. Combine the flour, baking powder and salt; add to the creamed mixture alternately with sour cream, beating well after each addition. Fold in blueberries., Spoon half of batter into a greased and floured 10-in. fluted tube pan. In a small bowl, combine the brown sugar, pecans if desired, and cinnamon. Sprinkle half over the batter. Top with remaining batter; sprinkle with remaining brown sugar mixture. Cut through batter with a knife to swirl the brown sugar mixture., Bake at 350° until a toothpick inserted near the center comes out clean, 55-60 minutes. Cool for 10 minutes before removing from pan to a wire rack to cool completely. Just before serving, dust with confectioners' sugar.