I think if you confronted and asked a lot of those progressive blog commenters about racism, they'd acknowledge that it's systemic and that no one is exempt from it; they're slurring people as ignorant and racist who openly and consciously buy into and act on racist beliefs

I guess I feel that that is kind of the problem -- if somebody only thinks about racism as systemic when challenged, and doesn't think about it in the day to day moments, including when commenting to a post that is specifically about racism, then they're perpetuating the racist system in all those other moments, you know?

Also, I think you're a lot more optimistic than I am about comments sections -- perhaps because, as you say, they're self selecting. I'm thinking about comments sections in the dailykos, and in salon's war room, and in think progress, and at feministing, where sometimes somebody gets called out for sexist/heterosexist/racist/ablist/transphobic language, but when it happens the response is often defensive. (This is not to say that there aren't awesome comments there, just that they're interspersed with hurtful ones.)