It's sort of like definition through the negative, though, to exclude results, as opposed to favoring search results of certain types, like Blekko's search engine does.
According to their official blog:
Perhaps the result just wasn’t quite right, but sometimes you may dislike the site in general, whether it’s offensive, pornographic or of generally low quality. For times like these, you’ll start seeing a new option to block particular domains from your future search results. Now when you click a result and then return to Google, you’ll find a new link next to “Cached” that reads “Block all example.com results.”
Users can click the "block" and a confirmation message is sent. Presto, that URL will not sully your results ever again.
Although, should you change your fickle mind, there is a list users can manage.
The feature rolls out Thursday and Friday in English for Chrome 9+, IE8+ and FF3.5+.