The black bars aren't drawn over the game. I think you mistakenly think that making them transparent would show more of the game content underneath, but there is nothing rendered underneath them.
If you want to fill the black bars with more content from the game - that's exactly what "Scale inner"/"Scale outer" do! I think I might have said the wrong one, I think "Scale outer" will show more content outside the viewport vs. "Scale inner" which cuts the viewport.