What cause bruises to itch? Is it bad to scratch them?

The main factors would involve breakdown of blood,release of multiple mediators: bradykinin,histamine,substance P,prostaglandins,healing process and this is intensified by excessive scratching ,which can turn into a viscous circle.Hope this helps.

Itching is a defense response that very likely evolved to facilitate circulation. In any healing wound, such as a bruise or even a broken bone, multiple processes are called into play to restore the injured tissues. All these processes -including nutrient addition, waste removal, structural integrity, apoptisis, cell renewal, and others - depend on intact and robust blood circulation, from both arteries and veins. Itching provides a mechanism for auto-mechanical promotion of circulation which promotes wound healing .You may then ask why do we sometimes itch where there is no healing happening. It is known, for example, that mosquito "venom" contains factors that prevent human blood from clotting as well as factors that cause itching. Why? Well, mosquitos evolved too, and did so in order to get an uninterrupted drink when they sink their probe into your arm. And then you itch the offended site...why?We really don't know..perhaps to regenerate the site for a second bite. Whatever reason, it must have benefited either you or the mosquito and possibly both.