Blogspot is owned by Google, so it will help your rankings. However, if you've started a blog in a niche you plan to stay in for a while, I encourage you to start a blog on your own domain. This way, you have control over your content. I've also had some friends tell me that they had their sites removed -- after being on there for a while -- for no reason. Using a free platform or whatever can be good for testing, though.
With Pinterest, I found that it can depend on the niche you're in. It seems like that there are only a few that are really, really popular. You also need to start building up followers before you start promoting stuff. You've got to follow the people that are following the leaders in a niche. Does that make sense?
Joey