If I crit. while using a Frostbrand's close blast encounter power, do I add in the "crit per plus" of d8 as well? Even though it wasn't used as a melee attack?
By RAW, probably not.
The encounter power for a Frost Brand Weapon does not include the Weapon or Implement keywords. This will prevent a lot of feats and abilities from affecting this attack. I don't think it is RAW to allow the critical bonus damage from Frost Brand Weapons when using its attack power. Generally you only include weapon/implement bonuses & properties in an attack if the attack includes the appropriate keyword.
As a DM I'd probably allow it though (since the weapon is clearly being used in the attack despite the lack of keywords - I still wouldn't allow its properties to be used for the normal attack and damage bonuses though).