Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 9-inch Bundt pan.

Cream the sugar and unsalted butter together in a large bowl with an electric mixer or stand mixer  until light and fluffy. Add the eggs, and beat until well blended. Mix in sour cream and vanilla extract. This is your wet mixture.

In a separate bowl combine flour, baking powder, and salt; This is your dry mixture. 

Add the dry mixture into the wet mixture and stir by hand with a spoon until just blended. Fold in blueberries; spoon 1/2 of the batter into the prepared pan.

Combine brown sugar,and cinnamon in a small bowl; This is your brown sugar mixture. 

Place half of the cake batter in the greased bundt pan, and sprinkle 1/2 of the brown sugar mixture over batter in the pan. 

Spoon the remaining batter on top, then sprinkle over the remaining brown mixture. 

Use a knife or thin spatula to swirl the nut mixture into the cake.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 1 hour. 

Cool briefly on a wire rack, then invert carefully onto a serving plate

You can dust with confectioners' sugar just before serving if you wish or just leave it naked! . ** See note below