Preheat your oven to 350°F (175°C).
Grease a 9x13-inch baking pan with butter or line it with parchment paper.
Cream together the butter and sugar in a large mixing bowl until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, mixing well after each addition. Stir in the sour cream and vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, mixing just until combined.
Gently fold in the blueberries.
In a small bowl, combine brown sugar, cinnamon, and chopped pecans for the streusel topping.
Spread half of the cake batter into the prepared pan, sprinkle half of the streusel topping, then add the remaining batter and the rest of the streusel.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool for about 10 minutes before transferring to a cooling rack. Dust with confectioners' sugar before serving.