I've just updated a program I used to use and uploaded it to sourceforge:

http://minivmac.sourceforge.net/examples/blacken.html

Blacken paints the entire screen black, and stays that way until a key is pressed or the mouse is clicked. The program itself is 3k. The archive includes the source. It is trivial modification of the Mini vMac utility TestSync, and I happened to be revising the Mini vMac extras just now, so it was simple to make Blacken.