I don't think Twitch Prime has the ad-free benefit anymore, they moved that into Twitch Turbo. The ads are also definitely stitched into the HLS playlist as segments, so even if you were to block them you would see a black screen for the duration of the ad, although Twitch does seem to be doing something where if a segment fails to load it tries to load the next one a few seconds later, so you might only see the black screen for 5s or so.