Preheat oven to 350 degrees. Grease 9 x 13" baking pan.
Combine brown sugar, pecans and cinnamon in a medium bowl and set aside.
Using a 2 cup liquid measuring cup, stir together the sour cream and baking soda. This mixture will expand.
In a large bowl cream together sugar, vanilla and butter until light and fluffy. Add eggs one at a time. Stir in flour and baking powder. Add the sour cream mixture and beat until well combined.
Spread half of the batter into your prepared pan. The batter will be thick. Sprinkle fresh blueberries and top with half of the Streusel mixture. Top with remaining batter and then with remaining Streusel.
Bake 35 to 40 minutes or until done when tested.
