I'm not sure blog spammers are very smart. They need to be more sneaky when trying to get a message through. Take the recent run of Mortgage Loan spam we've been getting on blogs.msdn.com. It looks at though they trying to post the same message to my entire post archive. The issue with this tacktic is that after the 3rd one it becomes very easy to identify, delete, and ban the user.
I'm surprised there aren't more spammers using some simple text analysis engines to parse what's on the page any make a coment that sounds closer to real based on the content. This would be sort of like embedding google ads on someones page that adjusted to the page content except that it would be text more related to what you wrote in order to get through the bloggers spam detection. It might also be better at getting people to click through.