Preheat oven to 350 degrees. Put butter and oil in 10 in. iron skillet and place in oven until melted. Meanwhile, in large mixing bowl sift together dry ingredients. In a small bowl, stir together milk, melted oil/butter, and vanilla. Pour into dry ingredients and stir until moisten. Pour batter into hot skillet and sprinkle blueberries on top. Bake for 50-60 minutes or until done.