How do you change camera safe area transparency? I'm trying to change red-circled area so that it is entirely black.  I don't want to preview what is outside of my camera.  Any idea on how to do this?
This is called the passepartout.  It can be changed or disabled (with the camera selected) under the Object Data properties panel > Display rollout.



Note: the Amaranth Toolset addon adds a passepartout slider to the Specials W menu when in camera view.