Preheat your oven to 400°F. Line a 12-cup muffin pan with paper liners or spray the wells with nonstick cooking spray. Set aside.

In a medium bowl, whisk together 1 cup flour, 1/3 cup granulated sugar, 1/3 cup packed brown sugar, and 1 1/2 teaspoons cinnamon. Pour in the melted butter and stir with a fork until the mixture forms coarse, sand-like crumbs. You can use your hands to pinch it into clumps. Set the streusel aside.

In a separate medium bowl, whisk together 2 cups flour, baking powder, and salt until well combined.

In a large bowl, whisk together the eggs, 1 cup granulated sugar, oil, sour cream, and vanilla until smooth and pale. Add the flour mixture and stir with a spatula just until no dry streaks remain. Do not overmix.

In a small bowl, toss the fresh blueberries with 1 tablespoon flour until evenly coated. Gently fold the floured blueberries into the batter.

Divide the batter evenly among the prepared muffin cups, filling each all the way to the top. A large spring-loaded ice cream scoop (about 3 tablespoons) works well for this.

Sprinkle a generous amount of streusel topping over each muffin, pressing it down lightly so it adheres. Place the muffin pan in the oven and immediately reduce the oven temperature to 375°F.

Bake for 20 to 25 minutes, or until a toothpick inserted in the center of a muffin comes out clean. Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.