Preheat the oven to 325°F. Grease and line an 8-by-8-in baking dish with parchment.

To make the crumb topping, combine the all-purpose flour, brown sugar, granulated sugar, salt, and ground cinnamon. Pour in the melted butter and mix with a fork until the ingredients come together. Keep the mixture in large crumbles; do not overmix.

Toss the blueberries and the extra 1 tablespoon of flour in a small bowl and set aside.

To make the blueberry cake, whisk the flour, baking powder, baking soda, salt, and ground cinnamon in a medium bowl.

In a large mixing bowl, beat the softened butter and granulated sugar using an electric mixer until light and fluffy. Add the egg and vanilla extract and 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 batter's top, ensuring that every surface is covered.