Hi there,
Unfortunately it is not possible to restrict the guest from reading specific blog post while having the ability to preview the blog post on the frontpage. The closest method that you can do is to make the privacy of the blog post to "viewable by registered user only" inside the composer as you can see from my screenshot here,
http://screencast.com/t/d0D4K2cEfS . However by doing that, guest user will not be able to see the preview of the blog post on your blog frontpage.
Hope you understand.