Preheat oven to 400 degrees F (200 degrees C). Grease 12 muffin cups, or line with paper liners. Whisk together the flour, sugar, salt, and baking powder in a large bowl.
Whisk vegetable oil, egg, milk, butter and vanilla extract together in a separate bowl until smooth, and stir the liquid ingredients into the flour mixture until moistened. Lightly stir in the blueberries. Spoon the batter into the prepared muffin cups, filling them 2/3 full.
Bake muffins in the preheated oven until they rise and the tops are golden brown, about 15 to 20 minutes. A toothpick inserted into the center of a muffin should come out clean.