What is the Card type of "Emblem Sorin, Lord of Innistrad"?
mtgmint lists it as Creature but I am not 100% certain.
It's not a real card. It's basically a token.
As a game object, it's an "emblem".
The relevant comp rules:
Sorin, Lord of Innistrad has an ability that creates these things (yes, multiples will stack). Dark Ascension booster packs contain an emblem token thingy to help you keep track of it. You aren't obligated to use the emblem card, just like you aren't obligated to use the official token cards to represent tokens.
The type is Emblem, more info on Emblems is available here. This particular emblem can be created by Sorin, Lord of Innistrad (gatherer link). Basically, an emblem is a reminder of an effect. So far, there are no ways to interact with emblems once they are created.