Period blood is frequently used in blood magick, as it's a very attainable source of blood. (It does come every month) Although some beliefs may view period blood as "dirty," it is blood nonetheless and is directly correlated with the cycle of life, making it very desirable for use in blood magick. It is also naturally occurring, and safer to obtain than making a new incision in the skin.
A woman's period is not disgusting, it is simply a part of normal life. This forum is for those 13 and older, and the members that visit this site should be mature enough to be able to discuss these topics. Pagans are known for "pushing the boundaries" a little when it comes to societies taboo's. (Ex. Doing religious rites in the nude, and performing public sexual acts) If these kinds of topics offend you, either don't look at it, or don't visit the site to begin with, as these topics will surely come up every now and then.