Blue prints allow you to get another exotic item that you already have. So in order to unlock the blue print you first must have that item. The first time you get the exotic item it unlocks the grimoire card and the blue print. Example let's say you just got the gjallarhorn for the first time. You get the card and the blue print. The blue print allows you to get as many gjallarhorn you want without having to earn it again.

As far as I know the post master will work as it does today. I don't know how it works once the taken king comes out. [i]stop screaming at me, I'm very sensitive. :)[/i]