You can use the same checkbox that you use in the server settings to blank the screen when you are connected to that computer. During the NoMachine session just go to the !M icon of the desktop you are connected to and open the Server settings. Enable screen blanking on the fly. Once you’ve finished, you can disable it and then disconnect.
Edit: We thought deeply about what you requested, and it can indeed be a very useful feature, something the user can do “on the fly”, when needed. And so this is the result (see attached).
We’ll be adding it to NoMachine 9. 🙂