The blanking of the screen can be used in conjunction with the automatic screen lock upon the NoMachine user's disconnection.

When the screen blanking feature is activated, the remote screen is blanked, and the remote keyboard and mouse are locked until the user is connected by NoMachine. I.e. operations made on the physical screen by the NoMachine user are not shown and the local user cannot interact with the desktop until the NoMachine user logs-out.

As soon as the session is disconnected, the remote screen is unblanked, mouse and keyboard are released. To avoid this, it's possible to activate the automatic lock of the screen, even if the user didn't lock the screen before disconnecting by NoMachine.

Screen blanking and automatic lock of the screen can be managed via User Interface as explained here:
https://www.nomachine.com/protecting-the-host-screen-during-a-remote-control-session

As an alternative, administrators may also enable/disable these features by editing the node and server configuration files (node.cfg and server.cfg) placed in:
/usr/NX/etc/ on Linux
/Applications/NoMachine.app/Contents/Frameworks/etc/  on macOS
InstallationDirectory\NoMachine\etc\server.cfg on Windows, usually C:\Program Files (x86)\NoMachine\etc

In node.cfg uncomment (remove the pre-pending # from the key name) and set the following keys to 0 to disable the feature or to 1 for enabling it:

EnableScreenBlanking

In server.cfg do the same for this key:

EnableLockScreen

Then restart the NoMachine server to make this change effective.

IMPORTANT NOTES

- For multi-node environments
Enabling or disabling screen blanking has to be configured on each of the remote nodes by editing the corresponding server.cfg.

- For Linux hosts
On some Linux distributions, gamma adjustments applied to blank the screen do not affect the cursor which remains visible. The local user, however, cannot control it with the physical mouse.

- For Mac OS X 10.7/10.8/10.9/10.10/10.11 and macOS 10.12/10.13
Mouse will be locked, but keyboard lock is not available on Mac.
This limitation is no longer present on macOS 10.14 or later.

- For macOS 10.12.6
The screen cannot be blanked reliably only on this specific macOS version. Previous OS X versions and macOS 10.13 do not exhibit this behavior.

- For virtual hosts
Screen blanking is available for physical hosts, it is not supported on virtual machines since it has effect on the physical monitor.

- Monitor brightness is automatically adjusted
The physical display is obscured as expected, but screen flickers and flashes making the desktop content to be shown intermittently. See also:
https://www.nomachine.com/AR01O00915

- Screen is unblanked while attaching/detaching monitor
When the monitor is attached or detached, the physical screen of the remot host can be unblanked for a moment while the graphic card resets its setting.

- For USB monitors
Screen blanking is not activated on physical displays connected over USB.

For NoMachine versions previous than v8:
- For Windows 10 20H2 hosts
At the moment, screen blanking is not available on Windows 10 20H2, which removes the possibility to rely on gamma ramp.
The related Trouble Report is:  https://www.nomachine.com/TR10R09864

- For Linux hosts
The locking of input devices when the screen blanking is enabled is not supported yet on Wayland.