If I export some of my old blog posts from one of my older blogs i have and delete them and import them into a new site/blog and give them different post titles. Will Google penalise me for that?

Google doesn't penalise for duplicate content, though it does get filtered out. Why not permanently redirect the posts on the old blog to the new one, ensuring that Google only have the one copy anyway - the one that you want them to have.

I'd do what gringo suggests. Put a permanent 301 redirect on the existing blog and point it to the page on the new blog, then remove the old blog when the new blog is indexed