If I were you, I would contact the florist and perhaps ask what she uses in her preservative. It is possible that the preservative you are making is lacking in a specific element that the daisy needs, or perhaps has something in it that the daisy is not reacting with properly.
If your preservative is in a bottle, maybe you can look up the ingredients on the internet and see what it is that is making it work so well!

I never realized how difficult a daisy could be! Then again, I've always kept them in a glass of water with one (1) aspirin in the water and let it sit for approximately 3 days before dumping it and cutting fresh.

Do you do your arrangements as a hobby, or are they gifts or something you sell? I would love to get into flower arranging, I recently planted some seeds out of a packet I purchased called "cut flowers" that is a variety of flowers I can eventually turn into table centerpieces over the season as they grow. :)

On a quick google search, I found the following website:
http://www.answerbag.com/q_view/2097885

It states that you should run floral wire up the stem and hook it back into the flower, as the stems are extremely delicate and can damage easily. Perhaps that can help you from having them wilt? As for the black, I am not really sure!