Depending on the configuration (as lower Graphics Controller as VboxVGA), it will show some messages in a box or it won't show anything else, just the black screen.[drm:vmw_host_log [vmwgfx]] *ERROR* Failed do send log message.
As I research about it, it is related to video emulation, and occurs when the resolution will switch to fit the screen. There is some workarounds but all I found is related to downgrade configuration or downgrade VirtualBox itself - but that seems horrible workaround to me! I want to workaround but end in a good point, with 3D acceleration and an updated host!
My own quick workaround is simple: just startup always in windowed mode, switch resolution later. Maybe I should make a script for it...
And I will suggest a workaround for developers: to put an option in the video settings tab to not switch resolution during startup - deactivate this automation. I know is not a solution, but a black screen like this is horrible when you need your system working, not everybody is skilled in to pick logs, go to text mode and write solution by hand. This option will prevent hang because any GPU request during boot is risky, depends on everything being ok.
If you google for this kind of trouble, you will see a lot of users trying to solve it for many years, they do a lot of things as reinstall Linux, some of them eventually "solve" and some just give up. With that option, will be easier to make it work before trying solutions that takes long time. Also, I don't think it's related to the host OS, it will happens in Windows and macOS.
Finally, keep working on this awesome, honestly I didn't find much of difference between VirtualBox and payed solutions as VMware, this project worth the efforts!