I can have very vivid mental imagery, I can imagine all sorts of fantastical things in detail in my mind on command. But that doesn't mean I don't see black when I close my eyes. It doesn't mean the mental imagery blocks what I can see normally with my eyes open. It is in your "mind's eye", so it is still triggering the vision parts of the brain, but almost like its on a different screen.

Similarly with imagining sound or music. I can almost perfectly recall some of my favorite music, I can compose a new piece (poorly) in my head based on music and sounds I have heard, but none of it will drown out an actual sound I hear around me.

That being said, if I am very mentally focused on some mental imagery it can still distract me from my real vision and or even sort of "replace it" without my closing my eyes. Sort of like the feeling you get when driving and zoning out thinking about something (but you are still safely driving).