The people with porn Tumblrs don't need to move, they just need an easy way to be found. Why not a retro, Yahoo-style directory? That's how lots of us found things before search engines got so good. Just start tumblrporn.com (lawyers permitting) and list all the blogs Yahoo doesn't want indexed, in categories. Sell ads. Profit!
As a person who actually USED Yahoo's original search engine, I think this is a wonderful idea, the law permitting, as you so wisely stated. And that it could actually be PROFITABLE as well is intriguing. BTW, you sig is both awesome and sadly totally correct.