Note that you can continue to use the Chrome extension to block sites, which I believe we rolled out before this feature. Get it at https://chrome.google.com/webstore/detail/personal-blocklist... And here's the blog post we did about the Chrome extension: http://googleblog.blogspot.com/2011/02/new-chrome-extension-...
Client-side spam blocking has been available for years. The primary advantage of having spam-blocking integrated into the search engine is that the user sees a full page of results for their query, even if some results were filtered out.
If the Chrome extension supports server-side filtering, then it must be sending some sort of "exclude results from these sites" message to google.com. It would be very nice if the message format was documented and official, even if it's something as simple as a JSON list. That would allow extension developers to add support to other major browsers, especially Firefox.