Ask PG: How hard would it be to implement a blacklist of domains for HN?

I'm guessing you mean a per user list. It would be easy technically, but I don't want to do it because it would take the pressure off making the site good.

Using an aggregation of the per-user lists to guide the evolution of the site-wide list would probably resolve that concern. It's also conceptually very similar to how submission flagging currently works here.

http://userscripts.org/scripts/show/32404 Hacker News filter for Greasemonkey seems to be what you want.

already exists. Can someone reply with the exact location/listings?

Sure.

http://news.ycombinator.com/item?id=499044