While I despise 4:3 video being zoomed in and/or cropped to fill 16:9 frames, it's equally bad to show the original 4:3 with blurred versions of the actual video filling the pillar bars. Just use black pillar bars to fill out the frame with the original 4:3 video intact at that aspect ratio. Jeez this isn't rocket science.
@lauren agreed, especially as that helps further processing handle the content as the particular end viewer wants it handled.
Black pillars let the end device know that there's no content there, so if a wrongheaded ( ;) ) viewer wants the display to zoom or stretch or whatever, it knows how to do it.
Blurred versions make the display think there might be content there so it can't help the end viewer reach the experience they want.
@volkris Right. For quite a span #YouTube would display 4:3 content in the standard 16:9 frame that they switched too many years ago, with black pillars, which was fine. Then fairly recently they switched to displaying 4:3 in a native 4:3 player so no pillars are even necessary, which is even better.
I now a bit about how YT operates technically from my times working inside #Google. It's pretty amazing stuff.