Preheat oven to 425°F (220°C) and line or grease a muffin tin.
In a bowl, whisk together flour, baking powder, baking soda, salt, and optional lemon zest.
In another bowl, mix sourdough discard, eggs, milk, melted butter, vanilla, granulated sugar, and brown sugar until combined.
Pour wet ingredients into dry ingredients and stir gently until just combined.
Fold in blueberries carefully without overmixing.
Fill muffin cups about 3/4 full and sprinkle with coarse sugar if desired.
Bake at 425°F (220°C) for 5 minutes, then reduce heat to 350°F (175°C) and bake for another 14 to 17 minutes.
Remove from oven and let cool in the pan for 5 minutes, then transfer to a wire rack.
Serve warm or at room temperature.