I went to order more blank Dominion cards from the BGG store and got the following notice:
Quantities limited to ONE per customer per direction from Rio Grande Games. Any additional orders of this to the same buyer will be cancelled and refunded.
Apparently this is the case for all of the Dominion promo cards, including the blanks. Elsewhere on the site it said this agreement with Rio Grande Games was effective January 1, 2012. It's not problematic to me to limit sales of the other promos, because how many Walled Village packs do I need? But what's the point of limiting the blank cards? I use the blank cards for making my own custom cards. Apparently now I can't do that anymore. This is extremely disappointing.
I don't know the official reason, but I can sure guess.
Promos are promotional. They aren't products. RGG gives them away, at a loss, in order to promote the game. They're promotional!
We don't want fans to be mad due to being unable to get a promo that was given away with a magazine or at a con or what have you; so, they are available through BGG. They are still promos though, they are still not products, they are still made at a loss.
The blanks may seem different but they aren't. There is no blank cards product. RGG loses money on each set of blanks that goes out. They exist simply because it seemed good to somebody to pair Stash with something on BGG, since Envoy and Black Market went together before that.
So it does not actually make sense to let people have all the blanks they want. It would only make sense as an actual product, that RGG charged for. It's unlikely to ever exist as a product though, due to lack of demand. You can probably buy them from people who have blanks they don't want though.