The Ouija board is useful, but inefficient, mostly because most people who use them don't know how to use them properly. The reason it doesn't work when the participants are blindfolded is because it works through the unconscious mind of the participants. The Spirits speak through you. Another possibility is that the disbelief of the experimenter interferes with the outcome, but this explanation is usually seen as a cop-out. A third possibility is that the Spirits are speaking their own language, similar to the "Angelic Language" or "Enochian" of Elizabethan occultists John Dee and Edward Kelly.
Try it with a pendulum. Make a circle of letters out of scrabble tiles, and use a pendulum to spell out the messages. Blindfold the person holding the pendulum, and see what results you get. It will work best if you are calling a specific spirit, and if the participants are open to the possibility of it actually working, rather than intent on disproving the phenomenon entirely. Also, performing the operation within a protective circle ensures that you are only contacting the spirits you want to contact. Your intent and your expectations do hold a powerful sway over these types of experiences.
In the end, though, it matters little if you are actually contacting literal external spirits, or your own unconscious mind. Either way, the information gathered is the same. Whether or not it is useful or helpful depends more on the individuals doing it than on the spirits contacted. Some occultists even see the spirits as aspects of themselves, but this psychological approach does not take away from the validity of the experience (and often adds to it.)
10 months ago
The book, _Aleister Crowley And the Ouija Board_, by J. Edward Cornelius, is a good place to start learning how to use the Ouija Board safely, effectively, and efficient