It is largely believed that the origins of the names “Mary Worth” and “Bloody Mary” came from a slight mix up of characters from history. Mary I, Queen of England, or Mary Tudor, who reigned during the Tudor period was also commonly known as “Bloody Mary”.
Her nickname of “Bloody Mary” became attached to her when she violently executed and burnt people at the stake for heresy throughout her 5-year reign as Queen of England.
She also was unable to have children and suffered two phantom pregnancies, this is why it is speculated that the variation involving chanting “I stole your baby” became tangled up with the legend.
There is another suggestion that the name “Mary Worth” was derived from a victim of the Salem Witch trials.
Another possible origin is the legend of Elizabeth Bathory, or Countess Dracula as she has been called. She was a Hungarian royal who was rumored to have killed young girls and bathed in their blood to preserve her youthful beauty. Of course her name was not Mary but somehow the stories of this cruel woman have been inserted into the legend.
An urban legend that says that anyone who chants the words Bloody Mary three times in front of a mirror will summon a vengeful spirit. This spirit has been reported to do a variety of things to the person who summons her, including killing the person, scratching their eyes out, driving them mad or pulling them into the mirror with the spirit – generally referred to as the spirit of a woman or even a witch. This is an old legend, but in 1978 a folklorist named Janet Langlois published an essay on Bloody Mary, which led to the tale becoming a popular slumber party ritual done by girls as well as boys. No one knows the origins of the Bloody Mary legend; over the years, she has been rumored to be anything from a witch that was killed for practicing witchcraft to a modern-day woman killed in a car crash, depending on what part of the country you live in.
http://www.supernaturalwiki.com/index.php?title=Bloody_Mary_(urban_legend)
http://www.scaryforkids.com/bloody-mary-legend/
http://www.snopes.com/horrors/ghosts/bloodymary.asp
http://en.wikipedia.org/wiki/Bloody_Mary_%28folklore%29
p.s. it's just a legend, don't be affraid