Preheat oven to 400°F (204°C). Line muffin tin and lightly grease the top.
Whisk together flour, sugar, baking powder, and salt in a bowl.
Mix oil, egg, milk (to reach 1 cup), and vanilla in a measuring jug.
Combine wet and dry mixtures. Stir gently just until mixed.
Fold in the blueberries without overmixing.
Divide into muffin liners, sprinkle with sugar, and bake 15–20 mins.
Cool briefly in pan, then transfer to a rack to finish cooling.
