Being possesed by a Jinn is usually the work of the Jinn himself, and is often caused by the possessed persons actions. Things like taking drugs or being a drunk can open one up to this, someone who frequents jinn infested areas, or sometimes even from purposefully or accidentilly harming a jinn. I once met a woman who became majnoon when she through a bucket of boiling water out her window, not knowing a jinn was standing there. the boiling water killed the jinn, and the brother of the jinn sought revenge on her by possesing her and mking her harm herself. Black magic is caused by Jinns as well, but they are jinns either under the control of a sahar or shayteen working with a sahar.