Line a baking sheet with parchment paper or a silicone baking mat.
In a medium bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
Cut the cold butter into the flour mixture with a pastry blender, fork, or your fingers until it looks like fine crumbs.
In a small bowl, whisk together the milk, egg yolk, lemon juice, and lemon zest.
Pour the wet ingredients into the flour mixture and stir until the dough just comes together.
Gently fold in the blueberries, being careful not to crush them.
Turn the dough out onto a lightly floured surface and knead it two or three times, just until it holds together.
Pat the dough into a 5-inch round about 1 inch thick.
Cut the round into 4 equal wedges.
Place the wedges on the prepared baking sheet, leaving space between each one.
Bake for 13 to 15 minutes, until the tops are golden.
Cool the scones on the baking sheet for 5 minutes.
Transfer the scones to a wire rack to cool completely before glazing.
Make the Glaze
In a small bowl, whisk together the powdered sugar, milk, and lemon juice until smooth.
Drizzle or spoon the glaze over the cooled scones.
Let the glaze set before serving. Sprinkle with a little lemon zest, if you like.
Video
Notes
Keep the butter cold: Cut the butter into the flour while it's firm, and if your kitchen is warm or the dough feels soft, chill the shaped wedges for 10 to 15 minutes before baking. Cold butter steams in the oven, which gives the scones their lift and tender layers; softened butter makes them spread and bake up flat.Mix with a light hand: Stir the dough just until it comes together and knead it only two or three times. Overworking it develops the gluten in the flour and makes the scones tough instead of tender.Check that your leaveners are fresh: Baking powder and baking soda lose strength over time, and stale ones leave scones flat. To test baking powder, stir a quarter teaspoon into a little hot water; if it bubbles right away, it's still good. To check baking soda, stir a quarter teaspoon into hot water with a few drops of vinegar; fresh baking soda fizzes.Cool completely before glazing: Let the scones cool all the way before adding the glaze. On warm scones the glaze melts and slides off instead of setting into a smooth finish.