She is only real if you choose to believe so. You see, if you convince yourself that someone appears, (Since in the ritual the bathroom must be dark) your eyes will play tricks on you and you may begin to think you're seeing a figure. In actuality, like I said before, your eyes are simply playing tricks on you. There is no such thing as Bloody Mary. If you strongly believe that she is non-existent, then what you think of as Bloody Mary will not appear. (:
Bloody Mary is a myth concocted from Queen Mary I who was a ruthless ruler. The myth says that you can taunt her by saying that you have her baby to get her to appear. This taunting comes from Queen Mary who had quite a lot of miscarriages. I hope that helps.