There are two different classifications, NSFW and Adult. NSFW is opt-in by the creator of the blog, and still allows the blog to be indexed by search engines. Adult is an automatic classification applied by Tumblr (previously the blog creator could also opt in to it, but in this announcement they say that will no longer be the case), and prevents the blog from being indexed by search engines.

See this table for details: http://www.tumblr.com/docs/en/nsfw