Unfortunately, the extension does little more than storing a list of domains on your computer and hiding the results from those domains. It's not tied to a web service and the blacklist is not saved to your Google account, so that you could use it from a different computer or another browser.
Matt Cutts says that the list of domains you've blocked is sent to Google. "We will study the resulting feedback and explore using it as a potential ranking signal for our search results."
Google SearchWiki used to offer a similar feature, but you could only use it to hide certain results. Blocking domains is more powerful and it will be interesting to see if it will become a regular Google search feature. I think it's too powerful and it might lead to unintended consequences: for example, some users might hide a domain just because a web page is not very helpful.