Yes, there was an instance recently where somebody did just that and the link to the article changed to the original link that was being commented on.

That brings up the question: what's considered blogspam and what's considered good commentary? Is that automated, or is there a judgement made by the moderators?

Will it though? I can think of plenty of blog posts that were commentary on links (albeit fairly extensive and/or insightful), that didn't get flagged as blogspam. OTOH, if all you do is editorialize on a link, with little to no insight, shouldn't it be flagged as blogspam? Perhaps one of the biggest reasons I don't blog very much is that I don't feel like I'm contributing anything of value by merely linking somewhere and going "isn't this cool?" I believe blog posts should have more substance.