Which Black magic spell would cause bleeding continuously?

While abnormal bleeding may possibly be caused by misused witchcraft, it usually has medical causes and anyone who bleeds continuously should consult a physician !

With that said, let's get to the magical part. I can think of many spells that could cause someone to bleed. The first one that comes to my mind is a healing spell gone wrong: if an inexperienced witch tries to heal someone and accidentally sends out excessive of inappropriate energy, that may disrupt the person's normal bodily functions and all kinds of symptoms may occur.

Bleeding could also result from deliberately harmful spells. For example, there is an old hex called the kicks and cuts cursewhich causes a person to frequently suffer minor accidental injuries (such as cuts and burns in the kitchen, falling down when walking, etc.) An unethical witch could also cast a spell to mess up a woman's menstrual cycle and increase her blood losses. With manipulative magic, it is even possible to force a person to self-mutilate him/herself.

These are just examples; witches can create new spells to fit their needs, and evil ones can invent very twisted ways to hurt people. Fortunately, magic has its own intrinsic laws, and harmful spells tend to return to the caster and bite his/her butt eventually. Most witches are good people who avoid hexing except for self-defense, and the minority who curse people without good reason have to suffer the backfire of their harmful spells.