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They're not going to give it any serious attention unless people actually file bug reports and complain. As far as I can tell, all tech support...
FYI, a temporary workaround here: http://forum.parallels.com/showthread.php?303168-Workaround-found-for-Black-Screen-on-Mac-10-9-3-Guest
Under Parallels Desktop 9 for Mac, upgrading a Mac OSX guest VM to 10.9.3 (official) results in a 'black screen' upon reboot. Multiple other...