Hashes. You have to know a hash to get content and the content.

Can't imagine that would work for preventing? Folks can just do a minor edit of the file to go around the blacklist. That said I imagine the solution for any hosting company applies to would be filecoin'ers too, on request of the government, certain hashes are publicized and update your node to blacklist them after the fact?

That's how all blacklists work.

Extra step would be using tineye.com or local machine vision to generate hashes for your blacklist based on visually similar images.