Amay Champaneria and Beverly Yang, Search Quality Engineers at Google, wrote:
You’ve probably had the experience where you’ve clicked a result and it wasn’t quite what you were looking for. Many times you’ll head right back to Google. 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.”
Once you click the link to “Block all example.com results” you’ll get a confirmation message, as well as the option to undo your choice. You’ll see the link whether or not you’re signed in, but the domains you block are connected with your Google Account, so you’ll need to sign in before you can confirm a block.
Once you’ve blocked a domain, you won’t see it in your future search results. (Side note: Sometimes you may have to search on a new term, rather than simply refreshing your browser, before you’ll notice the domain has been successfully removed.) The next time you’re searching and a blocked page would have appeared, you’ll see a message telling you results have been blocked, making it easy to manage your personal list of blocked sites. This message will appear at the top or bottom of the results page depending on the relevance of the blocked pages.
You can see a list of your blocked sites in a new settings page, which you can access by visiting your Search Settings or clicking on the “Manage blocked sites” link that appears when you block a domain. On the settings page you can find details about the sites you’ve blocked, block new sites, or unblock sites if you’ve changed your mind.
We’re adding this feature because we believe giving you control over the results you find will provide an even more personalized and enjoyable experience on Google. In addition, while we’re not currently using the domains people block as a signal in ranking, we’ll look at the data and see whether it would be useful as we continue to evaluate and improve our search results in the future. The new feature is rolling out today and tomorrow on google.com in English for people using Chrome 9+, IE8+ and Firefox 3.5+, and we’ll be expanding to new regions, languages and browsers soon.