Q.1
This version of chroma key depends on
Coming up with an effective definition of what's "blue" in the image
Being able to identify moon pixels
Coming up with an effective definition of the foreground person's image in the picture
Q.2
Think about using a red background for Chroma Key.
That wouldn't be cost effective -- red dyes are more expensive than green or blue.
That would work just fine, as long as we have a good definition for a red background.
That would not work because human skin has too much red in it -- it would be too hard to define "red" for the background but not the face