To put it bluntly, think I changed GRUB to text-only - which seems to work. But don't know if it's a safe, viable, acceptable long-term solution.
Detail:
In the VM, 21 would boot live/install from iso, but on reboot "blank" black screen (possibly sometimes with a non-moving single underscore cursor shown upmost left).
Control-Command(orAlt)-F3 would open a virtual terminal, under which logged-in and modified /etc/default/grub to show menu, rebooted, and from then on to get LM21, always had to select advanced options, select recovery mode, then from Recovery Menu select resume (Resume normal boot), then Mint runs OK (until next reboot).
Further web-searching found quite a bit of "black screen" boot problems re: Mint 21 and underlying Ubuntu 22, especially on Apple/Macintosh hardware. Usually focused on video drivers, and NVIDIA in particular. Tried nomodeset, and other experiments with a couple of individual drivername mode=0 or drivername.modeset=0 (like nvidia). Tried copying things which appeared unique to the recovery-booted-working-Mint Xorg.0.log KERNEL command-line into grub, like dis_ucode_ldr.
Nothing worked.
Until finally, found some grub commands referring to a text mode, experimented, and wound up so far using this final version:
GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=0
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_GFXPAYLOAD_LINUX=textboots as a "normal" quiet black screen which continues to open normal GUI Linux Mint 21.
noted: if add
GRUB_TERMINAL=consoleboot initially displays text of "Parallels BIOS ... boot from SATA drive 1...", and then blacks out as it continues to open normal GUI Linux Mint 21. (Not any faster or slower.)
and if instead of GRUB_GFXPAYLOAD_LINUX=text , using
GRUB_CMDLINE_LINUX="text" also works, but only if paired with GRUB_TERMINAL=consoleThanks for any comment.