01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Whisk together flour, sugar, baking powder, baking soda, and salt in a large bowl until well blended.
03 - Add cold, cubed butter to the flour mixture. Use a pastry cutter or fork to cut butter into dry ingredients until mixture resembles coarse crumbs with pea-sized butter pieces remaining.
04 - Whisk egg, sour cream, lemon juice, lemon zest, and vanilla extract in a separate bowl until smooth and fully combined.
05 - Pour wet mixture into dry ingredients. Stir gently with a spatula or wooden spoon just until dough comes together. Do not overmix.
06 - Gently fold blueberries into the dough, being careful not to crush them.
07 - Transfer dough to a lightly floured surface. Pat into a 1-inch thick circle, approximately 7 inches in diameter. Cut into 8 equal wedges using a sharp knife or bench scraper. Place wedges on prepared baking sheet, leaving 2 inches between pieces.
08 - Bake for 16 to 18 minutes until tops are golden brown and a toothpick inserted into the center comes out clean. Cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
09 - Whisk powdered sugar, vanilla extract, and 2 tablespoons milk in a small bowl. Add additional milk, 1 teaspoon at a time, until mixture reaches drizzling consistency.
10 - Drizzle glaze over cooled scones using a spoon or piping bag. Allow glaze to set for 5 minutes before serving.