I want to change a Blue sky into a night sky nearly black with very little change to the day light colour, colour correction won't give me black is there a way of doing this.
To add the stars etc do as George has said and place the foreground image/video on a lower track, you can then add the stars/moon image or video on a track above (see image below) and adjust the size/position for best effect.
Here is the result
Also here is a great place for getting background video loops such as the one I used above