Preheat and Prep: Preheat your oven to 350°F. Grease and line a 9×5” loaf pan with parchment paper.
Mix Dry Ingredients: In a bowl, whisk together the flour, cornstarch, baking powder, and salt. Set aside.
Combine Sugar and Lemon: In a large bowl, mix the sugar and lemon zest with your hands until fragrant.
Add Butter and Oil: Mix in the butter and olive oil until creamy, about 90 seconds.
Add Eggs: Beat in the eggs one at a time, mixing well after each addition. Scrape the bowl as needed.
Mix in Sour Cream and Lemon Juice: Stir in the sour cream, then add half of the flour mixture. Mix on low speed. Add the lemon juice, then the remaining flour. Mix just until combined.
Layer Batter and Blueberries: Spread 1/4 of the batter in the loaf pan to prevent blueberries from sinking. Gently fold blueberries into the remaining batter, then pour it into the pan.
Bake: Bake for 55 to 60 minutes, until a toothpick inserted into the center comes out clean.
Cool: Let the bread cool in the pan for 20 minutes, then transfer to a cooling rack to cool completely before slicing.