Preheat the oven to 400°F (200°C). Line a standard 12-cup muffin pan with paper liners or lightly grease each cup with butter or cooking spray.

In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt. Mix until everything is evenly combined, then set aside.

In a separate medium bowl, whisk together the melted butter, eggs, milk, sour cream, and vanilla extract. Whisk until the mixture looks smooth and creamy.

Place the fresh blueberries in a small bowl and toss them gently with 1 tablespoon of flour. This light coating helps distribute the berries throughout the muffin batter.

Pour the wet ingredients into the bowl of dry ingredients. Gently fold everything together with a rubber spatula until just combined. A few small lumps in the batter are perfectly fine. Avoid overmixing, as this can make the muffins dense.

Add the flour-coated blueberries to the batter. Gently fold them in, taking care not to crush the berries or overwork the mixture.

Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full. Sprinkle the tops with coarse sugar if you like a slightly crunchy finish.

Bake at 400°F (200°C) for 5 minutes. Without opening the oven door, reduce the temperature to 350°F (175°C) and continue baking for another 15–20 minutes. The muffin tops should look golden, and a toothpick inserted into the center should come out clean or with a few moist crumbs.

Allow the muffins to cool in the pan for about 5 minutes. Transfer them to a wire rack to cool further. Serve warm or at room temperature.