I've watched through https://www.youtube.com/watch?v=4Rb7_WdNlZY, the stream showing a "blacked out" St. Peters square. The "blacked out" square still has the street lights and Christmas tree decoration lit up and at some time (around -40 minutes from time of this post) you can see everything brighten up and the square becomes visible. It's easy to see the increased brightness of the already existing light sources, including a car driving over the square at the time of brightness adjustments.

This leads me to think it's most likely an automatic day/night setting for the webcam that failed but I'm not disregarding a targeted video blackout.