My various smart TVs have all have a Picture Off function. It's a menu button and third down from the top on the current Sony TV. I suspect this feature is far more widely distributed than anyone realizes, and this app is certainly a valid response to that.
Samsung - All Settings, General, Accessibility, then Picture Off; or Bixby "Picture Off On" (heh).
Sony - Remote button Quick Settings (gear), then Picture Off.
Vizio - Remote menu button, Timers, then Blank Screen.
LG - Full settings, Energy saving, then Turn screen off (I think?); or voice command "Turn screen off".
etc. But these are all atrociously unusable. I would say I don't even know what they were thinking, except I'm certain it's "if we give people a button to turn off the screen, they'll call tech support after they turn off the screen" — which, having worked in tech support, is completely valid.
It'd be nice if the HDMI CEC folks would add a command "turn screen off but maintain audio playback/passthrough", but as of HDMI CEC 1.4b, they assume that all streams are video, and so provide (and mandate) the Display On commands when a source is active.
