I would have thought a simple answer for well known virus/spam sources, to just blacklist any torrents from that site automatically?

Spammers would just add something moving into the fake video to get around that check. User verification is probably much more effective, and reduces the amount of hardware checking needed.
"This would be difficult for videos that are not-quite-well-known or completely original, wouldn't it?"
Obviously, but having a ranking system would be beneficial, and better than not having one.