It doesn't work quite so well with an iron as the foil goes on patchy. Even so, I do like this. I think it's a nice effect to use. Definitely something to consider for the actual finished ones.
When I was printing the cards in preparation for the experiment, I printed one on to a black stock to see if this would work. It would mean adding an extra piece of paper in the middle of the card that is white for the writing to go on, but decided to give it a try at least.
I like this too. I like black foil on black card though, looks pretty sexy. Had the same thing with the patchy foil though. Don't think this will be an option for the final ones, but certainly glad I tried it.