Preheat and prep: Heat your oven to 400°F (200°C). Line a 12-cup muffin pan with paper liners or lightly grease the cups.
Mix the dry ingredients: In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
If you’re using lemon zest, whisk it in now to release the oils.
Whisk the wet ingredients: In a separate bowl or large measuring cup, whisk oil, buttermilk, eggs, and vanilla until smooth.
Combine gently: Pour the wet mixture into the dry. Stir with a spatula just until most of the flour disappears. The batter should be a bit lumpy. Do not overmix.
Fold in blueberries: Add the blueberries and fold gently 3–4 times.
If using frozen berries, work quickly to avoid streaking.
Fill the pan: Divide the batter evenly among the cups, filling each about 3/4 full. Sprinkle the tops with coarse sugar if you like a crunchy lid.
Bake hot, then check: Bake 16–19 minutes, until the tops are golden and a toothpick comes out with a few moist crumbs. The centers should spring back when lightly pressed.
Cool briefly: Let muffins sit in the pan for 5 minutes, then transfer to a rack.
Serve warm or at room temperature.