If technology was advanced enough: An automaton that sweeps comments, keeps only ones that are relevant and deletes the rest. Both automatons and humans can comment, as long as their comment is relevant.
So you just have your bot Google the article's keywords and repost content it finds along with spam links. Or heck, just repost part of the article or another comment. Practically guaranteed relevance. Some bots already do this. Even this fictional ridiculously smart AI is not up tithe task.
in my greatly hubristic and obscenely self absorbed and self important opinion, the best way to have comments on a blog is moderation only, with a fake out instapost that confuses spammers, along with whatever pixie dust gmail uses to filter spam email.
If it has a spam link then it isn't relevant.
I'm not an expert on spam, but doesn't almost all blog spam contain a working hyperlink somewhere? So what if you just instruct commenters who want to post links that they should leave off the http:// or break the url some other way. Then have your blog scripts automatically fix all such broken url's into working hyperlinks. What is the likelihood a blog spammer would post a broken url?
Probably the same likelihood as a spammer deliberately constructing a custom spamming bot just for your website... but good point - It'll be difficult if the spammer posts mangled URLs as part of their spam.
I imagine you could use some idiosyncratic scheme; lots of different ways you could do it. Do blog spammers have time to customise their scripts for specific sites? Is it worth it? My guess is they tailor them to different types of popular blog software. And then have to hit a very large number of blogs. Sort of like email spammers have to send out enormous quantities of email.