I noticed this today. All the YouTube videos in every YouTube channel RSS feed I am subscribed to, have black bars on the left and right hand side. But opening the video in the actual YouTube page, there are no black bars.
My guess is that something has changed on YouTube’s end, so they code for embedding videos has to be updated accordingly.
I tried and I couldn’t get rid of them. I can stretch the video to whatever height, but YouTube is forcing the videos to that height and it seems it’s creating those bars, for whatever reason.