Preheat the oven to 350°F (175°C). Grease and line a loaf pan with parchment paper.
In a large bowl, whisk together 3 cups flour, 1 tablespoon cornstarch, 1 tsp baking soda, and 2 teaspoons salt.
In another bowl, beat 6 eggs, 1½ cups sugar, and 4 tablespoons room-temperature butter on medium-high speed until pale and fluffy, about 2 to 3 minutes. Add the zest of 4 lemons and ¾ cup fresh lemon juice.
With the mixer on low, slowly drizzle in 1 cup olive oil until the mixture is smooth and emulsified. Stir in 1½ cups Greek yogurt.
Add the dry ingredients in three parts and fold gently with a spatula until just combined. Do not overmix. Divide the batter into 2 bowls, stir in blueberries to one bowl, and let the batter rest for 5 minutes.
Pour the batter into the prepared pan and bake for 55 to 65 minutes, until a toothpick comes out with moist crumbs. If the top browns too quickly, cover loosely with foil.
Cool in the pan for 15 minutes, then transfer to a wire rack. While still slightly warm, wrap in foil or a clean towel to trap steam and keep the loaf tender.
Make icing by mixing 1½ cups powdered sugar, a pinch of salt, and 2 tbsp lemon juice. Drizzle over cooled loaf.
