Early in the NuTyX adventure, after installation, but while it was still running the "live" version, I used the VirtualBox command Host+F to take the VM to full-screen. And I ended up with a full black screen, where the mouse was not visible, right-clicking did nothing, and so on.
Using a VirtualBox menu at the top, I was able to leave full-screen mode and send it a shutdown command, and after I removed the .ISO file and restarted, everything seemed good again. It all stayed good as I continued doing setup, as documented in my previous post.
Until I started Firefox, and tried to go full-screen again. Another black full-screen, from which I could do nothing at all, except to bail out and quit the way I did the first time. The difference this time was that I could not restart the VM. Not at all.
I'm going to load the .ISO file back into the virtual optical drive and give it one more try from the beginning, but I don't have much reason to believe that it will work any better the second time. One thing that I will do is to try setting the video support to VBoxSVGA instead of the default VMSVGA.
That's about all I can change that seems like it might have something to do with the black screens. But my experience with other distros is such that the VM may simply lock up with the wrong video support. It's worth a try, though. If VBoxSVGA makes it all work well, I want to keep investigating NuTyX for a while longer.