Starting in 2011 both Google and Blekko started using blocking as a search engine ranking factor to adjust their search engine ranking results. No doubt that Bing won’t be far behind.
Blocking allows anyone who doesn’t like a search result from a site to block that site from ever appearing again in their search results. You won’t be notified that this has happened. Your web site will just never turn up in the results for that user again.
So what. A single user blocking your web site isn’t the end of the world. That maybe true but, Google is now using the aggregate blocking data to help rank their results. A site that is blocked by a lot of users will have its rank lowered.
This is pretty easy to avoid. Provide quality content and don’t try and create pages to spam the search engine.