In Real Life, hemophobia is an anxiety disorder characterized by -- well -- extreme fear of blood. Treatments exist, but the condition tends to be lifelong and is usually caused by childhood trauma. In fiction, being Afraid of Blood is much the same, though the symptoms tend to run a rather more colorful gamut from mere extreme discomfort or nausea to falling unconscious on the spot to awakening superpowered evil sides. Note that there is some scientific basis to this: physical reactions uncommon in most other phobias have been recorded in cases of fear of blood.