A witch doctor is defined as a sorcerer credited with powers of healing, divination, and protection against the magic of others in a community, and I use it interchangeably with cunningman.
A witch doctor heals and protects against witchcraft.
Black magick, as it's defined, is magick used with the intent to harm or breaking of the taboo.
The two terms are largely at odds with one another.
So that said, to be a cunningman/witch doctor, you must learn to heal and protect others from witchcraft in a community. To practice black magick, you must use your magick with harmful intent.