I know that some decks give you a few extra cards. Namely a double backer and I got a blank faces card in my silver split spades deck. I really can't think of anything to do with it other than making the face "disappear" with a snap change. What else can/do you use it for?
Some ideas:
1. Do a "blank fan" (reverse thumb fan in the left hand) with the blank card on the face to show the deck is all blank. Maybe follow up with a Hindu or overhand shuffle display as a quick convincer. Then execute the colour change of your choice and a normal fan to show the faces have reappeared.
2. Perform a two-stage colour change by "wiping the ink" off the face of a card (changing it into the blank card) and then "painting on new ink" (changing the blank into another card).
3. Apparently dissolve the ink on a card by secretly staining the tips of your fingers with black ink, displaying the ace of clubs, rubbing its face as you deposit the blank on it and showing the ink on your fingers as if you've just rubbed it off the ace.