I have run into a problem where I connect to a headless mac using Screen Sharing and I’m welcomed by a black square. I found an Apple discussion (now “archived”, so commenting is closed) describing the issue without a definitive answer. The discussion describes changing the display quality (view -> Full/Adaptive Quality) as well as toggling the scaling option.
Both suggestions above didn’t work for me. But, I have passworded the screen saver; blindly typing the password and hitting enter exits the screen saver and presents the desktop; problem solved!
I have noticed certain things like coverflow in iTunes or QuickTime movies also show a blank, black square. The initial black screen may have to do with hardware accelerated components like a screen saver that are not sent over the wire.