What about a whitelist for specific accounts? Opt-in lists are flexible but it would be a big effort to maintain reasonable lists across the 1000's of blogs. And frontends still have the challenge of selecting which one(s) should be default -- since the default view (say, for a non-logged-in user) should not highlight toxic comments to a reasonable extent.

In addition to a long tail, it seems like some users have a tendency to harass specific authors. It may only be an issue on certain blogs and not globally. Giving authors some power to moderate their blog greatly distributes the responsibility.