Preheat the oven to 220℃ (200℃ fan/425℉). Line a 12-cup muffin tin with paper liners or lightly grease the cups.
Whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, massage the lemon zest into the sugar, until fragrant and the sugar feels slightly damp.
Add the cooled melted butter and whisk by hand for about 30-60 seconds, or until the mixture thickens and lightens to a pale yellow. Add the ricotta and whisk until combined, breaking up any large clumps as you go. Whisk in the eggs, buttermilk, lemon juice, and vanilla until smooth.
Carefully fold in the dry ingredients until mostly incorporated and only a few streaks of flour remain. Be very careful not to overmix.
Toss the blueberries with 1 tsp of flour, then fold into the muffin batter until evenly distributed and no dry streaks remain. For frozen berries: add straight from the freezer — do not thaw!
Divide the batter among the 12 muffin cups, and sprinkle the tops generously with coarse sugar.
Bake for 5 minutes, then without opening the oven door, reduce the temperature to 180℃ (160℃ fan/350℉). Continue baking for another 15-20 minutes, until lightly golden and a toothpick inserted into the centre comes out with a few moist crumbs.
Allow the muffins to cool for 10 minutes in the tin, before transferring to a wire rack. Cool for a further 10 minutes before serving.