Just a quick reply to say that I have surrendered. Nothing has the desired effect. Some setting did cause slight differences in where the black screen began and ended, but that is all. I also tried adding "verbose" at the GRUB menu.
Based on what you (drs305) were saying about GRUB handing over to the kernel, I'm guessing that some particular graphics module is not compiled into this kernel, since GRUB_TERMINAL="console" does work (though it's too large & fast to read).
Thanks for all you help--I do appreciate it very much!