So, on a lark, I installed FreeNAS 9.10 Stable on some random hardware I had sitting around: A Broadwell Intel NUC and some random flash drives. Then I clicked "Check Now" and "Update". Guess what happened...
Yup! Same two issues!
On first reboot, I get the GRUB error. So I reboot with a USB drive made from the 9.10.1 ISO and now I have a running FreeNAS with the blank console issue.
So this happens repeatedly and predictably on totally different hardware with a brand new, fresh install. I imaging it ought to be easy to reproduce and hopefully to debug and fix. And I imagine it would be a good idea to pull that bad update before anyone else clicks Update and has the same experience.
Note, in response to some questions and suggestions I've gotten, that I've tried both UEFI booting and legacy and have tried a number of different flash drives for both install and boot.