Occasionally on older versions of OS X, connecting to a remote system via VNC will result in a "Black Screen" that is non-responsive. To fix this, kill the loginwindow
process:
$ ps ax | grep loginwindow
83 ?? Ss 0:00.89 /System/Library/CoreServices/loginwindow.app/Contents/MacOS/loginwindow console
$ sudo kill 83
loginwindow will automatically restart and the login screen will appear.
Thanks to Mac Mini World!