Blogger doesn't offer an option to detect spam comments. The only options you have are to add a captcha to prevent automated spam or to moderate the comments, but that takes away from the value of an instant feedback. Even if Blogger adds rel=nofollow to all the links from comments and you don't improve their ranking in the search engines, "bloggers" like Addison post the same spam comment every 5 minutes to promote some mediocre widget site. Because Addison posts the comments manually, he can enter the captcha correctly.
But Blogger could at least check if similar comments were posted to a single blog multiple times. Or use the Akismet model.
It's always surprising to see how a company that actively fights against web spam is defeated by some comment spammers that use cheap methods to promote their latest widgets.
