Step 1: Preheat your oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease it thoroughly.
Step 2: In a large bowl, whisk together the all-purpose flour, granulated sugar, poppy seeds, baking powder, baking soda, and salt until well combined.
Step 3: In a separate medium bowl, whisk together the sourdough discard (at room temperature), large eggs, melted unsalted butter or neutral oil, milk, fresh lemon juice, and lemon zest until smooth.
Step 4: Pour the wet ingredients into the dry ingredients. Using a rubber spatula or wooden spoon, gently mix until just combined. Be careful not to overmix; a few streaks of flour are okay.
Step 5: Gently fold in the fresh blueberries. If using frozen blueberries, do not thaw them; add them directly from the freezer.
Step 6: Divide the batter evenly among the prepared muffin cups, filling each cup almost to the top for bakery-style domes.
Step 7: Bake for 5 minutes at 400°F (200°C), then reduce the oven temperature to 375°F (190°C) and continue baking for an additional 15-20 minutes, or until a wooden skewer inserted into the center of a muffin comes out clean.
Step 8: Remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.
Step 9: Serve warm or at room temperature. Store leftovers in an airtight container for up to 3 days at room temperature or longer in the refrigerator.