Preheat the oven. Preheat oven to 350°F (175°C). Line a 9×5-inch loaf pan with parchment paper.
Cream butter and sugar. In a stand mixer fitted with the paddle attachment (or in a large bowl using a hand mixer or whisk), mix the melted butter and sugar until smooth and combined.
Add eggs and vanilla. Mix in the eggs and vanilla extract until fully incorporated.
Add sourdough and lemon. Stir in the sourdough starter, lemon juice, and lemon zest until smooth.
Add dry ingredients. Add the flour, baking powder, and salt. Mix just until smooth and no dry streaks remain. Do not overmix.
Prepare blueberries. Toss the blueberries with a small amount of flour until lightly coated. This helps prevent sinking.
Fold in blueberries. Gently fold the coated blueberries into the batter.
Transfer to pan. Pour the batter into the prepared loaf pan and smooth the top.
Bake. Bake for 60–70 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Glaze. While the bread is still warm, whisk together powdered sugar and lemon juice to form a smooth glaze. Brush or spoon over the top of the loaf.
Cool. Allow the bread to cool in the pan for 10–15 minutes, then transfer to a wire rack to cool completely before slicing.