Preheat oven to 350°F (177°C) and grease a 9×5-inch loaf pan. You can also line it with parchment paper for easy removal.
In a medium bowl, mix flour, baking powder, and salt. Set aside.
In a large bowl, beat butter, oil, and sugar until light and fluffy (about 4–6 minutes). This step helps create a soft texture.
Add egg yolks one at a time, mixing after each addition. Then add lemon juice, lemon zest, and vanilla extract, and mix until smooth.
Add half of the dry ingredients and mix gently. Then add sour cream and mix just until combined. Add remaining dry ingredients and mix until smooth. Do not overmix.
In a separate bowl, whip egg whites until stiff peaks form. This helps make the bread light and airy.
Gently fold the egg whites into the batter in two parts. Then fold in the blueberries carefully.
Pour the batter into the prepared pan and bake for 50–60 minutes, or until golden and a toothpick comes out mostly clean.
Let the bread cool in the pan for 15 minutes, then transfer to a cooling rack.
For the icing, mix powdered sugar, lemon juice, and milk until smooth. Drizzle over cooled bread.
