Combine the all-purpose flour, brown sugar, salt, and cinnamon. Add melted butter, and mix until crumbly. Place the mixture in the fridge until ready to use.
For the Blueberry Cake
Preheat the oven to 350° F. Grease and line an 8-by-8-in baking dish with parchment.
In a small bowl, toss together the blueberries and the extra 1 tablespoon flour, set aside.
In a medium-sized bowl, whisk together the 2 cups of flour, baking powder, salt, and ground cinnamon.
In a large mixing bowl, beat the softened butter and granulated sugar until light and fluffy. Add the eggs, one at a time, mixing until combined. Add the vanilla, mix to combine. Alternately add the dry ingredients and the sour cream, starting and ending with the dry. Mixing well after each addition. Finally, fold in the blueberries.
Pour the batter into the prepared pan and spread evenly. Sprinkle the crumb topping (streusel) over the top of the batter, making sure that every surface is covered.
Bake until a toothpick inserted into the center of the cake comes out clean, about 45 minutes.
Make sure baking powder is not expired, or close to expiration.
The tablespoon of flour keeps the blueberries from sinking to the bottom of the cake.