For the Crown is a 'deck-building' game in the sense of Dominion, not in
the sense of collectible games like Magic: the Gathering. You don't build
a deck before the game starts; you build up your deck during the game by
'buying' cards from a common supply. So unless you're using the terms
in a way I'm not familiar with, the Constructed/Limited distinction
doesn't really apply.
There are no blank cards in the initial release (I wanted to fit in as many
real cards as possible), but I don't see any reason we couldn't sell
blank cards separately in the future if there's interest.
