If you're shooting in B&W film, use the red color filter together with a circular polarizer to darken the sky really dark for more dramatic efx. In fact orange color filter is good enuff. In color shooting context, a circular polarizer will do. But do you know the best time to shoot in order to really obtain the true bluish blue sky? FYI, the best local time is between 2:45pm to 4:30pm. Give it a try.