Preheat oven to 400 degrees F. Line muffin cups with liners.
Mix lemon juice and milk and let sit for 10 minutes. Will “curdle” and serve as a buttermilk substitute.
Mix flour, sugar, baking powder, and salt.
Mix milk mixture with egg and vegetable oil and add to flour mixture.
Douse blueberries with cornstarch and fold them into the batter.
Fill muffin cups evenly. Sprinkle coarse sugar if desired.
Bake for about 20 minutes or until golden brown.