For every time the bad guy gets the girl, there are 100 or more failures. Blog spamming is like every other simplistic tactic, it fails for almost everybody who uses it. Of course, some people end up doing extremely well, for awhile anyway, with the tactic.
In any case, blog spamming is so 2003. The value of signing guestbooks is nearly fully over, and blog stuff is following, though again, it still works for some folks.
Oh, exceptions would be if you are talking about something non-competitive. Blogs or guestbooks would do nothing for “gambling” or “poker”, but they could help for “louisiana casino gambling” or something like that. Then again, they could also hurt. Too many spam signals can cumulatively raise a red flag.