1. Prepare your oven and baking sheet: Preheat your oven to 400 F (200 C). Line a baking sheet with parchment paper to prevent sticking and for easy cleanup.

2. Combine dry ingredients: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt until well combined. This ensures even distribution of the leavening agents.

3. Cut in the cold butter: Add the cold, cubed butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces remaining. The cold butter is key to flaky scones.

4. Incorporate wet ingredients: In a separate small bowl, whisk together the sourdough discard, milk, and lemon zest. Pour this wet mixture into the dry ingredients and mix gently with a fork or spatula until just combined. Be careful not to overmix, as this can lead to tough scones.

5. Fold in the blueberries: Gently fold in the fresh or frozen blueberries until they are evenly distributed throughout the dough. Again, avoid overmixing to keep the berries intact and prevent the dough from turning purple.

6. Shape the scones: Turn the dough out onto a lightly floured surface. Gently pat the dough into a round disc, about 1-inch thick. Using a sharp knife or a round biscuit cutter, cut the dough into 8 wedges or rounds.

7. Bake until golden: Arrange the scones on the prepared baking sheet, leaving a little space between each. Bake for 20-25 minutes, or until the tops are golden brown and a wooden skewer inserted into the center comes out clean.

8. Prepare and apply glaze (optional): While the scones are cooling slightly, whisk together the powdered sugar and lemon juice in a small bowl until smooth. Once the scones have cooled for about 10 minutes, drizzle the glaze over them. Serve warm or at room temperature.