I've only started to notice gray bars instead of black ones on top of adjusted images (from Forum posts, apparently related to overlay permissions in other apps), but the issue you're describing sounds a lot like the S8's video cropping feature. I've seen it go haywire on DirecTVNow, so if you activated it (albeit accidentally) at some point or are inadvertently doing so (the image adjustment icon pops up usually on the bottom right corner when tapping on the video) that might be why it's doing it. What helped for me in the aforementioned app was force-closing the app and restarting the video. Also try disabling video enhancements in your phone's settings to see if that's causing some of the issues.