Why do they use a blue screen (as opposed to some other color)?
Post #37 has an excellent link on the topic.
"Why do they use a blue screen (as opposed to some other color)?"
Sometimes it's green, but in either case, it is a shade rarely found in nature, so it is easy to isolate the actors. It is a technique called 'chroma key' and is sometimes referred to as a 'mat shot'. Same thing the weatherman uses.