I'd like to be able to overlay arbitrary solid rectangular blocks onto a composite video signal in much the same way a character/title generator overlays text characters onto the video signal. It's equvalent to putting masking tape on a TV screen to block out certain parts of the video. It's similar to, but simpler then, the character generators that allow people to title their videos.

What is the easiest way to do this? Do simple/cheap devices that combine two composite video signals exist?