At night, black with lots and LOTS of stars. Not only would the stars not be obscured by shining through the air and dust it contains, but our atmosphere has a slight glow even on the darkest nights. That would be gone.
The daytime would be equally intense. Without the atmosphere, we have no protection from the sun, which would look like a brilliant white light (the yellow light we see on Earth as it is is mostly due to the atmosphere's filtering) against a black background.
For an excellent example, it would look just like the sky on the moon.