if your blog.site.com gets high pagerank and different ip google will see it as different site and will think you are getting backlins from different site (well thats what i think but i might be wrong) I have UK based site and i have checked my competitors (big boys) and thats what they have done so i am guessing thats is the reason but like i said imight be wrong i am sure @testok will know better about this.
Not when it is on the same domain it wont
i have a site. (like domain.com)
i opened a blog on subdomain (like blog.domain.com).
I would have one link from your main site to your blog in your navigation. Then I would write blog posts on various topics, and when there is relevant information on your main site, link to it from the blog posts.