Preheat the oven to 350 degrees F (175 degrees C).

Make the streusel: Mix brown sugar, flour, and cinnamon together in a medium bowl. Cut in butter until mixture is crumbly.

Make the cake: Generously spray a fluted tube pan (such as Bundt) with cooking spray. Whisk flour, baking powder, and salt together in a bowl.

Beat butter in a large bowl with an electric mixer until creamy. Add sugar and beat until fluffy. Beat in egg and vanilla. Add flour mixture in two batches, alternating with milk, beating batter briefly after each addition.

Spread 1/2 of the batter in the prepared pan. Sprinkle blueberries over top, then add remaining batter, one spoon at a time.

Sprinkle streusel over top.

Bake in the preheated oven until deep golden brown, 55 to 60 minutes. Set on a wire rack to cool, about 30 minutes.

Invert cake onto a plate and dust with confectioners' sugar.