Like every other link building option the blog commenting is also getting saturated. But the opportunity to build will remain as long as there are do follow blogs. What I am trying to impress is the point that most of these do follow blogs are getting saturated by comments.If a page has more than 25 outgoing links well I dont think the link will do any good to you. It is always better to find links by yourself. Find those blogs that has limited links. Also I would suggest you go and post in those blogs that has no page rank. If the post is good it will receive pagerank. Even though at the time of posting the blog has no pagerank, it will have the rank in the near future. So never worry much about finding blogs with High pageranks.