Preheat your oven to 375°F. Grease 2 loaf pans.
In a medium bowl or stand mixer, whisk together the sourdough starter, sugar, eggs, melted butter, olive oil, milk, vanilla, lemon zest, and lemon juice until smooth.
In another bowl, stir together the flour, baking powder, baking soda, and salt.
Add the dry ingredients to the wet mixture and stir just until combined. Don’t overmix—this helps keep the loaf tender.
Toss the blueberries in a tablespoon of flour if you're using fresh blueberries (this keeps them from sinking to the bottom) and gently fold them into the batter. If you're using frozen blueberries you can mix them in without the flour. They work just as well as the fresh berries.
Pour the batter into your prepared loaf pan and smooth the top. Bake for 35–45 minutes, or until a toothpick inserted in the center comes out clean.
Let the loaf cool in the pan for 10 minutes, then you'll frost it by mixing powdered sugar and lemon juice to make a thick glaze. Slice once it’s cool enough to handle