Preheat oven to 400ºF. Spray 16 wells of two 12-count muffin pans with non-stick spray or line the wells with cupcake liners. Set aside.
In a large bowl, toss together 2 and 1/2 cups of the flour, baking powder, baking soda, and salt. Set aside.
In a medium size bowl, whisk together the brown sugar, granulated sugar, and butter until smooth. Add the eggs, buttermilk, and vanilla extract and whisk again until everything is combined.
Pour the wet ingredients into the dry ingredients and stir gently with a spatula until just combined. Set aside.
In a medium size bowl or a colander over the sink, toss the blueberries in the remaining 2 Tablespoons of flour. Carefully fold the blueberries into the muffin batter, being aware the purple color will spread a bit.
Spoon batter equally into prepared muffin pans. Place additional blueberries in the tops of the muffins if desired. Sprinkle each muffin with coarse sugar, if using. Bake muffins for 5 minutes, then reduce heat to 350ºF and bake for an additional 15-17 minutes until tops start to brown and a toothpick inserted in the center comes out mostly clean. Remove from oven and allow to cool completely on a wire rack. Muffins will stay fresh in an airtight container at room temperature up to 3 days or in the refrigerator up to one week. Muffins freeze well, up to 2 months. Thaw in microwave for 45-60 seconds before eating.
It is always a good rule of thumb to use room temperature eggs when using room temperature butter. To bring eggs to room temperature quickly, place in a bowl of warm water for 10 minutes.
If you don't have buttermilk, you can easily make your own: see my post about homemade buttermilk.
You may also use fresh blueberries or a combination of fresh and frozen.