Alright, I haven't heard anything new yet but you all do bring up some good points.
Yes the blank card can be anything you want it to be. Heck some Tarot Decks give you room to draw your own pictures and symbols. This is one of the ways you can use the extra card.
Yes, the card can mean nothingness. But what is nothingness? What do you mean by that? If you have a deep understanding of what nothingness really is than you can use that too.
The thing is, since the Blank Card never really started when Tarot was made (not when it was first made in existence but when it was started to be used this way) there isn't a right answer to how to use the Blank Card. "It's only true if you want it to be the truth." So there is no right answer or wrong answer, whatever works best for you is the right answer to you because you believe that to be the truth.