Preheat oven to 350°F (175°C). Grease and lightly flour two 8-inch round cake pans and line the bottoms with parchment paper.
In a bowl, whisk together the flour, cornstarch, baking powder, and salt. Set aside.
In another bowl, mix together the milk and lemon juice and let sit for a few minutes to curdle.
Beat the butter and sugar on medium-high speed for 4 minutes until pale and fluffy.
Reduce speed to low and add eggs, one at a time, mixing well after each addition. Stir in vanilla extract and lemon zest.
Alternately add the flour mixture and milk mixture in three additions, beginning and ending with the flour. Mix until just combined. Do not overmix.
Toss the blueberries with flour in a small bowl, then gently fold them into the batter.
Divide batter evenly between the prepared cake pans and smooth the tops.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pans for 10-15 minutes, then transfer to a wire rack to cool completely.