Heat oven to 400°F. Line a 12-cup muffin tin and lightly mist the liners.
Whisk flour, sugar, baking powder, baking soda, salt, and lemon zest in a large bowl.
In a separate bowl, whisk buttermilk, oil, egg, and vanilla until smooth.
Make a well in the dry ingredients. Pour in the wet and stir just until a few dry streaks remain.
Toss blueberries with a teaspoon of flour if using. Fold berries into the batter gently.
Scoop batter into cups, nearly to the top. Sprinkle each with coarse sugar.
Bake 16–19 minutes, until golden and a toothpick comes out clean or with moist crumbs.
Cool 5 minutes in the pan, then transfer muffins to a rack. Serve warm or at room temp.