Preheat oven to 350 degrees F.
Sift together flour, baking soda, baking powder, and salt into a bowl, set aside. Butter a 9-inch square baking pan, set aside.
In the bowl of electric mixer fitter with the paddle attachment, cream butter and granulated sugar until light and fluffy, about 4 minutes. Add eggs, 1 at a time until well combined. Add vanilla, and beat until combined. Add reserved flour mixture and the sour cream, and beat just until well combined. Fold in 2 cups blueberries. Spoon batter into prepared pan. Toss remaining cup blueberries with the crumb topping. Sprinkle the crumb topping over cake. Bake until golden brown and cake tester comes out clean, 50 to 60 minutes. Dust with confectioners' sugar before serving.
1 1/2 teaspoons ground cinnamon
1/2 teaspoon coarse salt
1/2 cup packed light brown sugar
1 1/2 cups all-purpose flour
12 tablespoons (1 1/2 sticks) unsalted butter, room temperature
In a medium bowl, combine cinnamon, salt, sugar, and flour; cut in the butter using your hands, 2 knives, or a pastry blender until well combined and crumbly.
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