Black screen around Virtualbox VM in fullscreen mode (Guest additions installed)

The guest additions cd needs installed within the client. You can do this from the client toolbar. Devices -> Insert guest additions CD image...

That will mount the guest additions cd image. Click on it to run it. Let it install. It will then reboot the guest operating system. When the guest restarts and you log back in you will be able to go full screen and the resolution will adjust automatically. You can scale the window whatever way you want by dragging the corners of guest window and it will auto adjust to whatever that resolution is.

Most probably the resolution in Virtualbox does not match your monitor resolution. Try to change the resolution inside the virtual machine.

I had a similar problem and did the following (with the Windows 10 VM stopped):

1. In the manager right-click and choose settings for the Windows 10 VM
2. Click on User-Interface options, at the bottom
3. Un-check/disable the option Show in Full-Screen/Seamless

This seemed too easy to possibly fix it, but it worked for me! No more black anomalies and such!

you need to install an extension pack which is available at:

https://www.virtualbox.org/wiki/Downloads