I just wanted to say thanks to all the people on Hacker News who asked for this option. We'll look at offering a "block site" option directly in the search results over time, but it takes longer to write, test, and launch that code. In the mean time, use this extension to clean up your own search results and tell us which sites you don't want to see in Google.
Is your block list something that will be synced so that it is available everywhere you use Chrome and eventually just synced with your Google account?