Reportedly it is easy to locate blogs of .edu sites where one could post relevant comments to get valuable backlinks since many of those blogs have high PR. Ideally one wouldn't have to be registered to post.

The search operator starts with site: .edu inurl:blog "post a comments"

Please assist me how to locate them.

There shouldn't be a blank between "site:" and ".edu". But if you do follow-up on this tip, you're spamming university sites, which you shouldn't (and it may well backfire).