Strongbird said:
Tumblr has started requiring accounts to view certain NSFW blogs...

Yep, that's because of the Safe Mode feature they implemented in June 2017. So now like other sites (FurAffinity, InkBunny etc) you need an account AND disable safe mode.

Also, NSFW blogs aren't indexed by Google or any other search engine, but I'm sure that's already been said.