WhiteRav3n makes a good case for precision in black magic. However, I've always considered it a crude and imprecise art.
Black magic deals largely with the physical and the bodily, because the so-called 'lower planes' - the portion of the astral plane reserved for negative energy beings and places, such as nightmares - are also associated with intense emotion, sensuality, and basic human potentiality.
To some witches dedicated to the black arts, this means integrating a sort of hedonism into ritual practice. Binge feasting, sex, killing (ostensibly, "hunting" or "fishing") - these are all potent generators of crude magical energy, and therefore perfect to perform before or during black magic acts.
That is, if you're into that sort of thing.
I don't like this mode of thought, because I think it unfairly dichotomizes magic into the 'good' and the 'evil,' disclaimers I've tried to avoid by not being conventionally religious. I prefer to think of magic as one giant grey blob with the occasional buzz of black and white static. What designates what is 'black' and 'white,' if not evil and good? Simple - order and chaos.
White magic reinforces the order of things. It abides by time-honored cycles man has relied upon for thousands of years to orient his day - the sun rising in the east and setting in the west, the ebb and flow of seasons, etc. - and generates 'positive' energy because of it.
Black magic, however, is antithetical. It runs west and east against the rise and fall of the sun and is best performed at night, when few people still awake are laboring benignly. It reverses and reflects the order of things, and in doing so, generates 'negative' energy.
Therefore, if binge feasting, sex or killing (again, "hunting" and "fishing," ostensibly) are not part of your usual day-to-day existence, they would indeed conform to this understanding of black magic. By doing them, you are stepping out of your routine and drawing the universe's notice. However, if they are, the very opposite would be true: performing acts of abstinence, chastity and mercy would produce the same effect. Magic is nothing if not relative.
This, combined with careful planning related to the moon cycles, the day of the week, and the planetary hour can generate potent black magic indeed.
I hope that helps. Blessed be.