Long gone are the days when the answer would be "Necromancy" and "Devil-Worship."
I am being honest here. If this comes off as a bit over-ethical, fine. Black Magic, by my definition is the focused intention to cause negative force based on selfish reaction. You intend to cause strife, pain, failure, harm, etc. simply because the target was successful, or won something, or got the job you wanted, or won an award, money, or got the car you wanted, or just wealthy, or prettier, or more buff, or just something like a misunderstanding caused you to just not like them- in any case, your outright jealousy or perception of the person caused you to create your vision of them as the Enemy, so you demand satisfaction to see their downfall. This can be simple curses, a jar, or any sort of intent to better yourself above them to outright calling upon the Abyss to swallow them whole. You demand Choronzon to get busy.
It is not fun being on the receiving end of this, all because someone just never understood you.