THE (EASY) SOLUTION: PATIENCE!
I had the same problem: when I started the virtual machine with EndeavourOS, the boot menu came up, then codes were running down - but then: black screen.
I checked several forum threads (like this) and tried different settings (like 3D acceleration etc.), but what helped in the end was simply waiting!
As I already wanted to give up (none of my attempts to change settings etc. seemed to work) and left the laptop and black screen besides me (while reading some WhatsApp), suddenly and unexpectedly the beautiful “Desktop” (?) of EndeavourOS showed up within my VirtualBox.
So forget about all the proposed complicated solutions, before you haven’t waited for the black screen to potentially disappear (after around 5 long minutes of waiting…).
Btw, I used a several years old laptop (Windows 10, SSD, but quite simple) and I made some of the standard settings for the VM within VirtualBox, but I think these changes were not relevant (for ex. I increased the used CPU from 1 to 2, RAM to 4096, I enabled 3D acceleration and then increased graphic memory to 256mb).
Hope this helps and that you’ll be pleased as I was, after patience had helped!