I never heard that before about blogs, but if it is true, it's probably because they are so easy to post a link to and don't usually have the moderation you find on forums.
I think there is some confusion in question, it should be no follow there in place of do follow. And do follow blog commnets have always great weightage in seo rather than no follow.
The value of this type of links (dofollow from blogs) was very much devaluation because a lot of spammers uses this type of promotion to make their sites more powerful in SERPs. When Google see that, they change the power of these links. That's the reason of weakness of these links.