It depends on who I want to have a discussion with. If the original poster (OP) is the blog author, then I'm probably going to comment in whichever environment

It depends on who I want to have a discussion with. If the original poster (OP) is the blog author, then I'm probably going to comment in whichever environment I think I can talk with the author. If the OP is not the author and I want to talk to the author, then I'd comment on the blog. If I want to rant/rave but not really to the author specifically, then to HN. But that's me - do what you want.