Finally, Blogger has added spam detection on their system. Well, i don't think it's really a spam detection as it relies on user's intervention to say that it's a spam or not, but i think it's much better than previous one.
All you need to do is to activate comment moderation on Blogger (go to Settings > Comments > Comment moderation). When you activate this feature, you will have a new tab called Comments besides Posting (under your Blog name), where you can moderate all comments that goes into your blog and you can mark them as spam or even delete them. This is a standard feature actually, but what Google does is to add an automatic spam detection for every comments that goes into your blog.
Since it's still being early developed, users will have to check their comment inbox to make sure that valid comments should not be marked as spam. For more information, please see Google Help