Hi Wander. Please use "Turn off or dim remote PC display(s)" option. You can set this option for each remote PC separately using connection settings.
After blanking remote display, Remotly will also block keyboard and mouse on the remote PC automatically.
If Remotly is running with administrator rights on the host PC the screen will be blanked in ALL situations during connection (desktop, UAC screens, screen saver and logon screen).
After disconnecting (or dropping connection) with remote display blanked, Remotly will automatically lock the host PC.
Please let us know if you have any doubts or questions.
PS. There is even a more advanced solution now. You can create a virtual display and completely disable physical screen by rendering desktop on a virtual display. Virtual display is not visble to anyone who has access to host PC.