Method
Preparation
Preheat your oven to 350°F (175°C) and grease a 9×5-inch loaf pan or line it with parchment paper.
In a large bowl, mix together the sourdough discard, sugar, melted butter, eggs, and vanilla extract until well combined.
Add the milk and stir to create a smooth wet mixture.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined to avoid overmixing.
Gently fold in the blueberries, lemon zest, and lemon juice, being careful not to crush the berries.
Baking
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
If the top browns too quickly, tent loosely with foil.
Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.
Notes
Cold, fresh blueberries minimize color bleed; if using frozen, fold them in straight from the freezer. Do not overmix after adding dry ingredients to keep the bread light.