for god sake!
Firstly of all, there is no such thing as a "dofollow". Either it's a link or it has a no follow rel.
Secondly,
use search engines!
Thirdly, I understand it is very tempting to
spam blogs that gives you a backlink but it is not worth it! The main point of commenting a blog post is
adding to the blog, being relevant, and ff your answer is interesting enough, people will follow your link. So if you want to drive traffic to your website, why not!
Spamming for the sake of a useless link, forget it! How many spam/useless comments do you leave on YOUR blog?