Preheat the oven to 400°F (204°C). Line muffin cups with paper liners (8 for big muffins, 10 for standard, or 22 for mini).
Lightly grease or spray the tops of the muffin tin with oil to prevent the oversized tops from sticking. High-quality nonstick pans may not require this.
Whisk the flour, sugar, baking powder, and salt in a large bowl.
Choose a measuring jug that holds at least 1 cup. Add the oil to the jug, and then crack in the egg. Pour in enough milk to reach the 1-cup line on the jug (this should be about ⅓ cup of milk). Add the vanilla extract and whisk until everything is combined.
Add the wet ingredients to the bowl with the dry ingredients. Stir gently until the batter comes together. Be careful not to overmix! The batter will be thick, see tips below.
Gently fold in the blueberries.
Divide the batter between muffin cups. If making big-topped muffins, the batter will come to the tops of the paper liners. Sprinkle a little sugar on top of each muffin.
Bake muffins 15 to 20 minutes or until tops are no longer wet and a toothpick inserted into the middle of a muffin comes out with crumbs, not wet batter. Transfer to a cooling rack.
To store, place them in a plastic bag, seal, and store at room temperature for up to 3 days. To freeze, wrap them tightly in aluminum foil or place them in freezer bags. Freeze for up to 3 months.