I have the exact same issue.
I’m using manjaro 20.1 cinnamon, and now i installed windows 10 to dual boot.
I reconfigured grub, but that didn’t help. As you said, i can interact with the grub menu while the screen is black.
Same issue on my installation - Manjaro 20.2.
I’ve tried the advice from @stephane, even I’ve enabled the sound on GRUB start (GRUB_INIT_TUNE="480 440 1") but no luck. On boot, I can hear the sound and there is the black screen as well. If I navigate with the arrow keys, if feels like something is happening in the background and when I make some combination with them, it redirectes me to the BIOS menu.
Bump for the visibility. Maybe someone more experienced will have additional questions.
Thanks for this, it sorted the issue of grub not showing, now I have to figure how to add my windoze drive that I use for gaming to grub to show up again as that has disappeared from the menu…