Black magic is essentially any mortal magic that breaks any of the Laws of Magic. It usually involves the cost of life and/or free will.
It's so easy to use because Magic come from the heart, from your feelings. Black Magic comes from lust, fear, and anger—things that are easy to feed and make grow. Harry's magic was harder because came from a deeper truer and purer source—harder to tap, harder to keep but more elegant and powerful. Belief in magic and confidence in oneself are essential to making magic work, otherwise it will fizzle out. This is particularly true for actions like killing someone with magic.
The worst kind of black magic gives off a nauseating, greasy almost empty feel. Fire could be a purifying force in its non-physical aspect—fire could consume and destroy dark magic if used with that intent. It was the weapon of choice for most Wardens. 
In the seriesEdit
- Main article: Fool Moon
- Main article: Turn Coat
- Main article: Changes
In Changes, Bob looks at photos of a shipment of Metacapacitors, which is code for ritual objects. He says that they're objects of dark and dangerous magic, which is obvious from angles and the imbalance, they are meant to be destructive. They are meant for Human Sacrifice. The slaughter of an innocent. The Red Court is planning a destructive act of high Black Magic.
- Main article: Ghost Story
In Ghost Story, Harry Dresden recalls his first encounter with Black Magic the day he came home early from school, he sensed and energy that hung in the air like "cloying, oily perfume". It felt wrong. Elaine Mallory was sitting still, quiet, Justin DuMorne stood at the kitchen doorway. There was a straightjacket on the table next to his copy of The Hobbit. Justin ordered him to sit, and when he didn't, Justin sent a blast of Kinetomancy at Harry who used the force to propel him through the window for an escape.