[I found a .edu blog that is also a PR10 that I can comment on, but of course, links are Nofollow. I know others will disagree, but I think there is value in NF links, as I have had a few hundred PR5+ blog posts done by hand that ranked 13th me for an easier keyword in less than two weeks. I just started using the PR10 .edu blog and I’m curious to see how effective it is. I do those myself writing and being relevant so they get approved and stick.]
Blogs, or websites, do not have PR. PR is assigned to the individual pages of a blog. When you post comments on a popular blog the posting page usually ends up in the archives or under a category eventually, and it is that page’s PR that weights the value of the link juice that is passed if any.
And yes, pages that are nofollow still offer value since it is only Google that does not pass link juice. Other search engines, like Bing and Yahoo, may still pass link juice on nofollow backlinks.
Also, even if Google does not pass link juice, if it is a popular blog, you will still get users clicking through and targeted traffic is always valuable.