Preheat oven to 350 degrees F. Spray a non-stick 9" bundt pan with baking spray with flour.
In large bowl, beat the butter and granulated sugar with electric mixer until light and fluffy. (about 3 minutes)
Beat in the eggs, one a time. Add in the buttermilk and vanilla and beat on low until combined.
In a small bowl, whisk together the flour, baking powder and salt. Fold into the wet mixture and stir until just combined. Fold in the blueberries.
In a small bowl, combine the brown sugar, cinnamon and pecans.
Spread half of the batter into the prepared bundt pan and level. Sprinkle the brown sugar mixture in a strip in the middle of the batter, staying away from the edges of the pan.
Spread the remaining batter into the bundt pan on top of the brown sugar mixture and level top with a spatula.
Bake in oven for 55 - 60 minutes, or until a toothpick inserted into the center comes out cleanly. Take out of oven and cool in pan for 10 minutes, then invert onto serving platter and allow to cool before serving. Dust cake with powdered sugar before slicing.