Shameless plug - I've been working on something similiar, but with a different aim.

https://github.com/nmurray1984/porn-blocker-chrome-extension

I found that the hardest problem to solve was the prevanlence of false positives. Even if you have a low false positive rate, it's still very likely to have an image blocked regularly just due to volume.

For that reason, the focus of my plugin is - for users that are OK with it - contributing URLs that are not NSFW but have been blocked. The extension includes a right-click menu option to do so.