If anyone anywhere on the web happens to link to a blog that is set to block search engines, then that link can be picked up and you can end up in searches via the other site's link. The search engines honor the privacy settings on your blog, and will not crawl it, but all it takes is one link from an outside source to a post or something you said in a post or page, and you can then end up in searches via that outside link.
The only way to prevent it is to keep the blog set to private (from the very beginning - never public). Once it has been public, it make take up to a year for the blog to disappear from searches entirely and if there are still links to your blog from other sites, then you may never get off of the search engines.
My old blog here that was set to private 2 years ago still shows up with 1 link in Google. In Yahoo, because my old blog is linked to from various other sites, I still show up with 12 links.
Even Bing, which didn't go live till 2 years after my blog was set to private has 11 entries (links from other blogs and sites).
Once someone links to you from another site, your cover is forever blown.