Okay. But even if I am wrong about what is going on here, it does no harm to re-install the GRUB boot loader even if it doesn't need it. That is, if it's done correctly. So good luck to everyone with this. Anyway, here is the thread were I recently got the idea that a hung system at GRUB time with only a blinking cursor is related to the old and well-known "GRUB _" thing...
In that case, re-installing GRUB worked. So for now, I'm viewing this as just a new variation and manifestation of and old well-known thing.
P.S.: Lately, people have also been describing black screen hangs (so to speak) well after GRUB is finished and during the time that the kernel is loading or later when the system is spinning up (so to speak). Obviously, re-installing GRUB for that would be folly. So with these hangs at a black screen and nothing else except maybe a cursor, it matters were along in booting it occurs. You will know the difference I think.