Google has announced a Chrome extension that enables the ability to block sites, called Personal Blocklist. Google has admitted to using the blocking feature in their ranking algorithm. If you and thousands other block a certain site, it may raise a “Flag” to google and they could potentially use that as a ranking factor.
To use this new Google Feature you will need to download the plugin for Google Chrome here.
Once you have the plugin installed you will see:
Here is the whole right up from Matt Cutts @ Google’s Official Blog: