Do multiply links from different websites on a blogspot count as if they were links from different domains? Maybe every next link gives less weight. For example, if my website has got 100 links and 50 of them come from blogspot's sites is it OK?
It is OK but if it were to benefit you then they should all be from trusted blogs like labnol.blogspot.com I'm worried if these links would kill your blog rather than benefiting you because these days there are lot of spam on blogspot.
Every link towards your site will be helpful, but it will be batter if u have it from reputed blogs with related theme. try to put fresh content on your blog so it will help more to get SERP for your site.
I agree that all links are "good" but you will need a few thousand PR0 links to get a decent PR4 or PR5. Consider that and build yourself a market plan.