Not only do these exist, there are significant institutional practices that exist to facilitate the sharing of detection signatures between companies and between companies and various governments.

It's good to prevent the re-sharing of child pornography, but a lot of companies don't like to talk about the systems because they're very much the same systems that the same companies are busy telling the content industry that they can't build to enforce copyrights. So they're generally kept pretty hush-hush by a kind of gentleman's agreement.

I think general copyright is actually a thornier problem and this secrecy is unnecessary. CP is just something you can block if you see it. Tracking down exactly which uses of a copyrighted work are intended to be allowed and which are not is really much harder. And I think that's a legitimate argument, especially because it recasts the copyright debate in terms of compromises that make sense: if clearing a bunch of rights is confusing, maybe certain types of compulsory licenses are more attractive. But that debate is for another thread...