In fact, you can get software just for that purpose. For example, Comment Kahuna is very popular, and very good at finding high-PR blogs.
However, most blogs with high PR have that Pagerank on their main index or home page. Sub pages will generally have much lower pagerank.
When you post a comment on a blog, chances are your comments aren’t appearing on the home page where that post is. And even if it is, it will likely “roll off” the home page long before you get any of the pagerank benefit of that home page.
Commenting, or blog posting is a great for getting links, and is a central part of my own strategy. Also, the net value that’s passed on from a high-pr blog will always be higher than that of a lower pagerank blog.
That being said, the fact is that that when you comment on a post that’s on the home page of a PR4 blog for instance, you aren’t getting the same ‘value’ that you would for that same link on a static PR4 page, and in fact might not be getting much value at all.
Blog commenting should be used as one component of your linking & traffic strategy. Just be aware that you may not be getting as much value as you might have thought.