How can I blank one display without changing the desktop layout?

Keep the display connected and use software dimming or a supported screen-control command. Disconnecting it removes its desktop space and can move windows or change which display is main.

To darken the image, use software dimming on the intended display. An LCD backlight may stay on behind a black image. The monitor may also stay awake rather than enter standby.

For a hardware screen action, open Settings > Displays > [display] > Additional Settings… > Display Connections and set Display menu on/off switch behavior to Screen Control, if a compatible controller is available. The display’s App Menu switch can then darken the screen while retaining the connection. The switch requires a controller that supplies the relevant screen state.

DDC screen control depends on the monitor: some models do not support it reliably or cannot wake again through the same command. Keep a working screen and a manual recovery method available when testing it. DDC Power Off is a separate action that requires manual power-on. It is not a substitute for a reliable screen toggle. See the display header and connection switch for the choices and confirmations.

If you want a fully dimmed image restored after sleep, review full-dimming restoration. The screen may become visible again after wake, even while its desktop stays connected.