Here's a thread from June this year where we discussed this feature:
"Block all listing from this site" - message in the SERPs [webmasterworld.com]
It's an option you get when you are signed in, click on a search result and then quickly return to the SERP for another choice. Google (and other search engines, for that matter) have long been differentiating between these "short clicks" and "long clicks", where it seems like the user became engaged on the destination page for a reasonable amount of time.
The blocking option gives them a new layer of information/signal to confirm that the short click was because the destination site seemed crappy.