As unusual a request as this is, I must ask, why? Tap tempo flashing, I understand. LCD or mini-displays? You can dim the mini-displays, but not turn them off entirely. Main screen displays your current preset and scenes, or is used for editing from front panel. Main screen remains on by default. Not sure why you need the lights off, unless you're wearing night vision goggles at home or are adversely affected by light. Sorry, that's what I imagine, not to be taken seriously...
Choose:
Set Up > FC Controllers/Footswitches > Enter > (Page over to) Config > (Use NAV Down or Scroll Down to) Appearance > Mini Display / Ring Brightness > Edit > (NAV Up or Scroll Up) > Exit > Exit.
You can also choose within FM9-Edit:
FC Edit > FC Config > Edit > Exit