Having never used either, I dont really think you can go wrong either way. If you were going to use Blackfire, I would reccomend layering a coat of Paste Glaze or liquid (even paste) Souveran over top and im sure you'll get a shine that will knock your socks off.
I can't speak for Zaino but I can tell you that Blackfire is stunning on white. Do a search for Blackfire or Zaino on white and you'll come up with some great pictures. I created a thread awhile back with pictures of Klasse, Blackfire, and Zaino examples on white. It should give you something to start on. Check out the links in my signature. There are good examples of Blackfire in there too. :xyxthumbs