What a pain in the nether regions when spammers decide to use your blog as a vehicle for their crap advertisements. My blogs are moderated, so I see every comment before they are posted. I only twigged that my blog was being spammed when I saw the same comment twice for different postings on the same day (that spammer was enthusiastic). It seems (naïve, perhaps) that any comment with a URL, either in plain view or in an HTML <a> tag, is likely to be spam comment, and showed be examined closely.
Suggestion to Blogger team: include a option to mark a comment as spam, just like Gmail, and use the crowd to identify spam comments.