Working magic for blind people is not easy.
I once tried using sponge balls for blind people, but since they can not see the transfers, they are constantly wanting to check the magician's hands, and they always want to open and check what is placed into their own hands. If you try to place a number of balls into the spectator's hand as if it were only two, for example, the blind person will immediately open his hand to check how many you put there.
Try having another magician perform the effect for you while your eyes are tightly shut, and you are trying to figure out the trick. Blind people are naturally alert to people taking advantage of them, with change, etc., and are not easy to misdirect.
Mental tricks can be effective, but often rely upon verification by trusted sighted friends of the blind person.
Having the blind person pour water from one styrofoam cup into a cup in his other hand and using slush powder to make the water disappear might be a possibility, but again you would find the blind person wanting to feel inside the cup to see what happened, and they are often more sensitive to changes in weight and balance.
My Routine for the Blind is really great in the right situation, and involves making the blind person look like the star--as if he/she is psychic. It uses an impromptu code so that the blind person is made a conspirator with the performer, and then fools the blind person using Brainwave, and Out of this World. But it only can work if you are working for only one blind person, with his/her friends around for support and verification.
The blind person needs to hear their gasps of amazement in order to buy into the whole thing, and be sure that people aren't taking advantage or making a fool of him. This type of routine doesn't really play for more than one blind person.
David Williamson has a great funny story about trying to use my routine on an elder blind person, and how badly it went wrong. David Acer wrote it up.
So it isn't an easy task. I would love to find better answers to this problem, and think that it is wonderful topic.
Love to see that book, Eduardo. Mara's Project sounds really interesting. Is it available in English? I would be glad to find a Spanish copy if not.