Lunar's BlackOut feature can do that: https://lunar.fyi/pro#blackout

The feature is especially useful if you want to turn them off in software and keep them still connected to the Mac so that you can turn them on at a later time using a hotkey for example.

If you simply want to turn them off completely, just like you would press the physical power button of the monitor, then Lunar can do that by sending a DDC toggle power command, but there are some caveats to be aware of:

    works only if the monitor can be controlled through DDC (most monitors work)
    can't be used to power on the monitor
    when a monitor is turned off or in standby, it does not accept any commands from a connected device

I recently wrote a deep dive into how I tried to find a better way to turn off these screens using private macOS APIs: https://alinpanaitiu.com/blog/turn-off-macbook-display-clams...

The latest code on the master branch of displayplacer supports this. Clone the repo and run `make` to build it. Run `displayplacer "id:34686E82-0CED-DF86-AFC7-AA1A8EB5CFC0 res:1440x2560 enabled:false"` with your screen ID and it will set the screen to all black plus remove the screen from the view of the OS.