Muffins: 1 ½ cups all-purpose flour ¾ cup white sugar 2 teaspoons baking powder ½ teaspoon salt ⅓ cup vegetable oil 1 large egg ⅓ cup milk, or more as needed 1 cup fresh blueberries Crumb Topping: ½ cup white sugar ⅓ cup all-purpose flour ¼ cup butter, cubed 1