Heat oven to 400°F. Line a 12-cup muffin pan with paper liners.
Toss blueberries with 1 tbsp flour in a small bowl. Set aside.
Whisk flour, sugar, baking powder, baking soda, salt, and lemon zest in a large bowl.
Whisk buttermilk, melted butter, beaten eggs, vanilla, and lemon juice in a separate bowl.
Pour wet ingredients into dry. Stir gently until just combined; the batter should be lumpy.
Fold in the floured blueberries with a few gentle strokes.
Scoop batter evenly into the liners, filling almost to the top. Sprinkle 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 to finish cooling.