Over at The Groovy Age of Horror they're discussing how the Bloody Mary myth is possibly rooted in more ancient beliefs about the power of mirrors to tell the future in a practice called "scrying."
The blog features a quote from Mirror Myths (an entire forum devoted to these legends!):
Divination rituals such as the one depicted on this early 20th century Halloween greeting card, where a woman stares into a mirror in a darkened room to catch a glimpse of the face of her future husband, while a witch lurks in the shadows, may be one origin of the Bloody Mary legend.
(Bibeau's Note: I found the photo above in a different location, so it's not necessarily what the text is referring to. But it seems to illustrate it nicely.)
Halloween by Lesley Bannatyne, a detailed and fascinating account of the holiday, has quite a few references to mirror divination. Girls would scan all kinds of reflective surfaces for bridegrooms on the horizon. Two passages (both with Irish origins) stand out for their originality and the danger they posed of incredibly ugly accidents:
Lie down on your back by a well on Halloween and hold a mirror over your head so that you can see a reflection of the bottom of the well. If you are to marry, the picture of your future marriage partner will appear in the mirror.
[G]irls believed... if they walked down the basement steps with a lit candle in their hand and looked over their shoulder in a mirror, they'd see the face of their future husband.
Scrying is divination with mirrors, but also crystal balls, and other kinds of optical media . This article claims to provide step-by-step instructions to help you make your very own scrying mirror. Please don't use it to do anything that might drown you or break your neck.