Preheat oven to 425°F (220°C) and line a 12-cup muffin pan.
Whisk flour, sugar, baking powder, baking soda, and salt in a large bowl.
Whisk eggs, buttermilk, oil, and vanilla in another bowl until smooth.
Pour wet into dry and stir gently just until combined (don’t overmix).
Toss blueberries with 1 tbsp flour and fold into the batter carefully.
Divide batter into muffin cups (about 3/4 full) and sprinkle coarse sugar if using.
Bake 5 minutes at 425°F, then reduce to 350°F (175°C) and bake 14–16 minutes more.
Cool 5 minutes in pan, then move to a rack and cool before serving.