Weird Black Border

That's a) not black but blue and b) is not the sheet but the canvas, which you can cahen in Preferences > Canvas > Background

That's just the background.

More to the point, though, you don't need to remove it to make it not be visible, you simply need to press the Home button or otherwise move the music back to the top of the window. That background only displays if you for some reason move the page outside the visible window as you seem to have done.