There is a Greasemonkey script[1] which does blacklisting. It works by sending "-site:example.com" with every search query. The script does a lot more cleanups like removing link tracking and ads, so if you don't feel like using all the functions, look in the code and check out "RemoveBadLinks()" function. Maybe you want to comment all other public functions in init().

[1] http://userscripts.org/scripts/show/79742

Wouldn't that reach a limit pretty soon? I've commented on this before, but there's a max character length for the search box.

MaxLenght of the input field is 2048, so you can put lots of domains in it. Of course, it's not a complete solution but it works to some extents, specially because it filters out stuff on server side and not with a css' display:none.

There are a few grease monkey scripts that the same thing. I wrote my own that does this and also fetches more results to fill in the gaps left by the "This site has been blocked" entries, but sadly I forgot to back it up before I upgraded hard drives and wiped the old one.

"There are a few grease monkey scripts that the same thing."

Not quite. This sends the info to Google whereas the GM scripts just CSS display:none the links. In theory this is different since the optimist in me hopes that Google will do something positive with the "downvotes".

there's this:

https://addons.mozilla.org/en-US/firefox/addon/custom-google...

but for cross-browser, it's best to roll your own:

http://radleymarx.com/blog/better-search-results/

Personally, I'd prefer Google change the Google Custom Search pages to be more like traditional search. If I could have image and map links, I'd probably never revert back to regular search.

BTW - my personal filter has 172 sites so far...