The black bars fill where you duplicate the video content onto a lower layer and magnify and blur it is one of those very stupid effects that are used because to a toddler’s mind they seem like a great idea. But it provides zero benefit. It reduces compression efficiency by leeching off bitrate and generates a constant distraction for the watcher’s attention through pointless movements over the majority of the picture, which is especially bad because human vision is more sensitive to movements towards the periphery and because the zoom amplifies the actual movement energy of the source video. It also blurs the perceptual boundaries, making it more difficult to clearly and easily identify where the actual informational video content ends. This is even worse on videos that are intended to be fully informational and not artistically playful, like for example this one:
In short, it is added effort to make a video a PITA. (In German this is called Verschlimmbesserung. Could be translated as worseprovement.)
Here is a good video demonstrating how easily attention can be diverted: https://www.youtube.com/watch?v=Ahg6qcgoay4
(It is a somewhat different case, but it is about moving elements drawing attention.)
UPDATE: I recently encountered a typical result of when people follow some trend thoughtlessly; a 16:9 video with mixed media content in which all 4:3 (so not even 9:16) aspected content had black bars fill, but cinemascope content did not have it for the black bars on the top and bottom. Suddenly black bars are totally fine. Why? Because everybody leaves them black.
This is just one example for many common inconsistent habits that show how people can be programmed into behaviors if they don’t question them.
UPDATE 2: Related issue with what Twitter has been doing for a while now: https://twitter.com/Dowlphin/status/973490387443769350
I also made a short clip pointing out the somewhat arbitrary, inconsistent behavior and how it has to do with herd following and how it’s not happening if no one starts it: https://www.youtube.com/watch?v=okkIOgqki50
UPDATE 3 – 2020-08-16: UNBE-FRICKIN-LIEVABLE! This is collective derangement!
Vertical video, put a fat frame around it to further reduce vertical image resolution/size, add the notorious blur fill to a high-motion scene, zoomed(!)… and then flipping the blur content horizontally!!! (Actually the video itself is flipped and the blur cloning didn’t follow.) … And the image isn’t even perfectly vertical, but rotated slightly to the right!
UPDATE 2021-02-04: Eventually a theory occured to me that maybe people are doing this in order to prevent the GooTube algorithm from labeling Fair Use content as ripped off and demonetizing the video, but this would only work on videos with large black bars anyway and also I have seen a lot of content like that that was uploader-original, so that would have to be a pointless habitual thing then. – And also, still, to this day, no one ever responded to my inquiries about why they are doing this stuff, so I have to assume they know they’re doing something stupid but want to continue. (I’m really working with an utter lack of external information here.)