You can get ranked for any keyword by doing only blog commenting, yes I did that for various websites. But its very hard to find huge dofollow links for blog commenting. Blog commenting has no comparison with article submission or social bookmarking. You get 100 social bookmarks or 100 article submission for only $4-6 but you can hardly find 10 blog commenting link (quality) for $10. Thanks
high PR dofollow blogs are difficult to find, specially with high PR pages where we can comment, luckily I have some.
Why is PR important to you? Why is it so difficult to find do follow blogs? What do you use blog commenting for? Are you looking for traffic or are you link building to poisition your site in the serp's? Or is it the PR you are after so that you can in turn sell links from your site?