It's my perception that there were so many policy changes about adult content after Yahoo! acquired Tumblr that I don't know what's currently the real deal. What's Yahoo's definitive policy about NSFW blogs at this date?
Are adult tumblrs blocked from 3rd party search engines (e.g. Google)?
If you sign-up for the first time right now, do you have to "opt-in" to view adult tumblrs while browsing Tumblr logged-in?
Best Answer
Reading posts
According to the documentation, users need to opt-in to be able to find posts from NSFW blogs with Tumblr’s search:
It seems that this setting was available at tumblr.com/settings/dashboard, called Safe Mode.
Now it’s directly on the search page, a little padlock icon:
This works only for logged-in users. Anonymous users can’t search for NSFW content.
Writing posts
Tumblr (still) allows NSFW content:
NSFW blog owners should flag their blogs:
If a blog is flagged as NSFW, it can still be crawled by search engines:
It seems that once there was a setting that implied that all adult blogs (there was a differentiation between adult and NSFW) would have to be blocked, but according to a staff’s blog post, this was a mistake and only applied to a small set of spammy NSFW blogs.