You're right, but it can not increase your PR even it can increase your traffic. Why it can't increase your PR? Because your link / comment at nofollow blog will not be followed by the spider of G (G is Google). So.. commenting at dofollow blog is better.

How to check a blog whether dofollow or no follow? If you use Mozilla Firefox as your browser, just install a plugin named NoFollow Plugin. You can search it in G by typing this phrase "NoFollow Plugin for Firefox"

Thanks..