I've noticed that Pinterest is now so over-stuffed with falsely attributed AI-art that it is spilling over to Facebook communities, where people will celebrate water colors and oil paintings attributed to painters who never did them.

The first thing an AI-blocker extension should do is thus to cross reference all images with Pinterest, and just NOT show anything that is available there. Harsh, but the user might then choose to reveal a hidden image on a case-by-case basis. Deviant art is another community that is now chuck full of AI images. People don't repost from there very often, as artists there tend to claim ownership of the images themselves, but it might be nice to block out their entire catalogue as well.

It'd be able to tell as easily as all other AI-recognition software is able to tell.

I have been very interested in an extension like this myself recently, and I believe it is fully possible.

Isitai.com is currently working on a paid extension that allows to to spot AI images directly in your browser. Though it is not meant to block them before they show up.

An aggregated common block list might be necessary to make what we want happen. Or a desktop-app that scans for AI signature patterns in the entire frame buffer.