In a case like this Google would count them as seperate links OR at least they should do, I mean Google owns blogspot I think they'd know that each blog is in fact maintained by a different person and not the same one and thus each link is like a seperate vote from a seperate person. Even in other similar cases the links should be counted as seperate. Many people use shared hosting and share 1 IP address it would not be fair for Google to discount what are seperate votes/links just because they're on the same IP. But, if for example one person registered mulitple domains all under the same nameand tried to get their rank up by sending links all from domains he owned, Google would be able to spot that the links are all coming from one guy who owns all the domains.
As for your original question. I wouldn't be so quick to say that blogs help rankings. They DO NOT help rankings, at least not directly. A blog helps a domain in 2 ways 1) It provides a place to put interesting content that people will actually want to link to, if your content is good enough and you promote to the right social networks you can get a lot of attention and a lot of links to your blog. If the blog is hosted on your domain, those links will benefit your whole site. BUT, the success of your blog and how many links it gets is wholly dependant on good content, if you can't write something interesting related to your niche, don't bother. 2) Because a blog allows you talk in long reams of text you can endup ranking for some bizarre long tail searches that aren't very competitive. Thus you end up getting lots of small trickles of traffic from lots of longtail searches. Understand that a blog does not automatically donate a better rank, a better rank comes from the indirect effect of your content generating references through links, which then help you rank better.