I saw a blog which has posts that are around PR 3. Shall i put my comments in such posts and get links to my blog through that? Does google give any penalty for linking from a same blog so many times? I wont comment spam that blog though.
Does the blog use do follow? I think you won't get any penalty for multi comments as most of "real blogs" do have regular visitors.
Like axlarry said, it will only be of slight search engine benefit if the blog enables do follow. However, just by posting on one blog multiple times, you won't see your rankings jump.
Read this one : http://googleblog.blogspot.com/2005/01/preventing-comment-spam.html To be short, find blogs without rel="nofollow".
In my opinion you should just comment on blogs that rank in Google acroos the board and don't concern your self with the no follow issue. Commenting on blogs related to your site topic is usually a good idea. But dont comment on the same blog from different sites.