Both blue and green screen are called chromakey.  Chromakey green generally
has an advantage over blue.  More people wear blue than green in their
costumes, making it the choice of most chromakey users.  Of course, if their
is green and blue in the scene (besides the screen) this won't work.
There isn't actually any rear screen projection involved.  The blue (or
green) is electronically replaced by source video from a pre-recorded
videotape, videodisc, camera, etc... and is primarily used in situations
where it is either more cost effective to use stock footage, or would
otherwise be safer than performing dangerous stunts which may be created with
a combination of animation, models, and movie magic.
Ron Hebert, Executive Director
Broadcast Training Institute
Screen-L is sponsored by the Telecommunication & Film Dept., the 
University of Alabama.