This only works if blog spamming is a compute and/or bandwidth intensive activity.
Publish a standard file or URL like robots.txt that blogs can provide in order to notify a robot that comments are moderated.
By market forces, robots should check this so they do not waste their computing time and bandwidth spamming moderated blogs.
Alternatively. Remember those copy protection things in computer games of the 80’s? “Please type in the third word of the second paragraph on the eighth page.”
This could be used in blog captchas too. Make the instructions of which word(s) to find just convoluted enough to avoid machine parsing, and you’re good to go. The instructions can then be generated on the fly.
Proof of concept to follow.