What kind of sky blue background would you like to see? There are various ways of creating sky blue background murals. You could use trompe l'oeil technique to paint a sky blue backgroundlight on the ceiling to create the impression of a view through a window. This will obviously be quicker than painting an entire ceiling, though that is quite easily done too, even over large expanses. For a more ambitious illusion you can paint an entire room to resemble an outdoor space, with the sky blue background effect also covering the upper part of the walls.