>cashiers have never so much as glanced at my cards
Depends on where you shop. Some stores (particularly big box stores -- Best Buy comes to mind) always ask to see the card and ID. If they do, it's a showstopper for it to not be real. There's NO WAY a store with that policy would let you buy something with an anonymous black "card".
Not only does this scream "FRAUD!", it almost certainly WILL be used for fraud, almost immediately on release. Think of how easy it would be to allow a server at a restaurant to "steal" someone's card -- photo and a swipe, and POOF, you can use their card the next time you go somewhere that doesn't look at the card. How long will it be before stores that DIDN'T ask to see it before start asking, and specifically disallowing the use of these fraud-enabling devices?
Regardless, there's no way I'd risk getting to the point of purchase somewhere and finding out at that point they won't accept my card. And I AM part of their target market: I probably have 8 cards in my wallet, maybe 10. It's complicated, but they all have a unique need that makes me carry them.
I'd love to have something like this that "just worked," but I doubt this would be it without orders of magnitude more education and many layers of security protection to prevent fraud -- probably enough layers to be an impediment to people actually signing up to use it (receiving physical mail being required to register a new card comes to mind).