In the case of black bars, I have a Galaxy S9 phone which has a wider aspect ratio than 16:9. I spend a LOT of time viewing 16:9 content, and in the case of full screen images that are solid colors, SOMETIMES you can see that the black bars are slightly different from the 16:9 area. I stress slightly because I'm just not that anal, and it's so rare to see.
As for the C9, I do plan on using it a LOT with black bars as I play lots of PC games in a 3840x1600 resolution giving it an Ultrawide presentation, so my top and bottom will look like movies. Why do I do this? Because it's less taxing than full 4k, doesn't have scaling artifacts, and gives games a wider FOV without the fish eye lens effect.
I much, MUCH prefer games have a black bars than scaling up from a lower resolution to full screen which looks blurry and ugly to me.
One example is Greedfall on PC. At native 4k, the game dips below 60fps a lot of times with max settings. With my custom res of 3840x1600, it pretty much stays locked at 60. Black bars don't bother me at all. I rather like it since they're so black on this tv and my room is dark so the tv disappears into my background, so the bars don't look like bars. Looks more like my TV is Ultrawide.
Mad Lust Envy