Big linux noob ahead, so please be kind if I don't exactly understand everything I'm talking about here.
I installed Ubuntu on my father's old macbook 4.1 a few days ago, and it worked just fine. Today we tried to install Elementary OS (multiple times) but it always got stuck on a black screen with a blinking white line - holding shift during boot did nothing.
After searching around a bit, I decided to try and reinstall grub2 with the tool "boot-repair". Didn't fix the issue, so I tried to reboot to reinstall Elementary without installing updates. I noticed that there now was a third option in the apple boot-menu: "EFI boot" (aside from "Windows", the one that would normally start ubuntu, and then my usb installation media). I thought "what the hell" and tried the option.
Here I am greeted by a quick screen that says
Failed to open \EFI\BOOT\grubx64.efi - Not Found
Failed to load image \EFI\BOOT\grubx64.efi: Not Found
Failed to open \EFI\BOOT\MokManager.efi - Not Found
Failed to load image \EFI\BOOT\MokManager.efi: Not Found
From here I can simply choose "elementary os" and the computer boots as it is supposed to.
I also tried some other things in the midst of things, including following this guide for Debian, and applying it to ubuntu.
Sorry if this has already been answered