A black screen is issued when users connect with NoMachine to the physical (or virtual) deskop of a remote machine and the following conditions are all met:

- NoMachine server is installed on a VirtualBox machine.

- 3D acceleration (hardware-accelerated graphics) is enabled in the VirtualBox settings.

- Any of the NVDIA GeForce mobile GPU is present.

The black screen problem doesn't occur with the desktop version of the GeForce GPU or when a different graphic card is in use on the laptop.

The suggested solution is to disable 3D acceleration in the VirtualBox settings.

Another possible reason for getting a black display is:

https://www.nomachine.com/AR09L00814

A black screen is displayed when running virtual desktops on Unity 3D or GNOME 3 and proprietary drivers