Looks like it might have been something else because they say they fixed it in the latest update Parallels Desktop 12 for Mac 12.1.2 (41525) : Resolves an issue with virtual machines stuck at black screen on booting
But they said the same thing in update Parallels Desktop 12 for Mac 12.1.0 (41489) : Resolves an issue with virtual machines stuck at black screen on booting
and also in the update Parallels Desktop 12 for Mac 12.0.1 (41296) : Resolves an issue with virtual machines booting into a black screen
So hopefully it really is fixed on this latest update.
Last edited: Jan 10, 2017