I can't really think of a lower form of advertising than blog comment spam. I think regular spam is less annoying. At least you know they're just carpet bombing the globe with 25 mill emails. You were not singled out, just careless with your emailaddress. Blog comment spam is different. I am not talking about the robo-spam I have previously mentioned - that's just more of the same, and it was actually more of a prank than actual spam. No, recently I have been receiving an increasing number of lowgrade comments to my blog, vaguely topical to the posts they comment on, but from the looks of it mainly there in order to promote the attached URL. These things are very hard to do automatically I should think. Unless of course it is just a very specific search on google and then some 'blogware URL pattern recognition' on top of that to find places to post.
They seem a bit too topical to be that automated, but maybe that's just my skills at cold reading that are failing me.