Preheat oven to 350 degrees. Grease and flour a fluted tube or Bundt pan.
In a large bowl, cream butter and sugar until light and fluffy, 5–7 minutes.
Add eggs one at a time, beating well after each addition. Beat in vanilla extract.
In a separate bowl, combine flour, baking powder, and salt.
Add dry ingredients to creamed mixture alternately with sour cream, beating well after each addition.
Fold blueberries gently into batter.
In a small bowl, combine brown sugar, pecans if using, and cinnamon.
Spoon half the batter into prepared pan. Sprinkle with half the brown sugar mixture.
Top with remaining batter and remaining brown sugar mixture.
Cut through batter gently with a knife to create a swirl effect.
Bake 55–60 minutes or until a toothpick inserted near the center comes out clean.
Cool in pan for 10 minutes before removing to a wire rack to cool completely.
Dust with confectioners’ sugar just before serving.