Until Jeff answers definitively this is a 'poll' type question. When you find a site that is blocked, please post it as an answer here.
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I agree however with the "what content exactly" part of the question.
If you refuse a post because of a content, explain what it is, or this is just frustrating. I once fought with a stupid post office form for 30 minutes to understand what "invalid character" was in what I wrote, because it wouldn't tell me.
Personally, I favor downvoting and deleting all answers that are just in the form of a web link, since one goal of SO is to have all necessary information on the site, and there's always the chance that information elsewhere will be deleted or modified so it's no longer appropriate. It's also possible that people will read SO offline sometimes, much as there's ways to read Wikipedia offline.
This is true of the bleep answers I've seen: they're links to links to potentially incorrect and potentially temporary information. I don't see any use for bleep on this site other than feeling superior to others, and find any banning quite reasonable.
This is not to say that people shouldn't Google for information before replying, or provide links to outside sources, only that people should include enough information in the post itself to be useful.
Note: I do think preventing submission of the first version of this answer because of a textual mention (not a link) of a site formed from the initials of "Let me Google that for you" may be going overboard. At least on MSO, where we might be expected to discuss these things.
I wouldn't expect Jeff to post a blacklist of excluded sites - why make it easy to find alternatives to the blocked ones?