Black magic is substantively no different than ordinary magic. Both types of magicians are making a choice for earthly goods (power) and are willing to solicit/manipulate the supernatural to obtain the fruits of their choice. The black magician is set apart by the vehement suddenness and ugliness of his approach. The black magician doesn't mince words and doesn't toy with appearances: He has his goal in mind and he takes the most direct route he can find.
And the most direct route is through the dark powers. As Chesterton observed above, there seems to a primordial instinct that the darker powers address mundane issues (which isn't surprising since the darkest power–Satan–is the author of the confusion in human nature that results in the lust for earthly power). Black magic, accordingly, is willing to appeal to the black spirits, like the devil, or to perform disgusting rituals, like Constanzo's human sacrifices, in order to obtain earthly advantages. Its practitioners are willing to reach into the pit of existential ugliness to obtain an earthly advantage. As Thomas Howard pointed out in his discussion of Simon the Clerk in Charles Williams' novel All Hallow's Eve: "Simon is more than curious, and we find that his great experiment is to find the word or words that will furnish the man who utters them with power. It is, of course, the attempt of all black art . . . The trouble with the black arts is that they attempt to do this by ripping the fabric of things: obscenities, outrages, perversions–whatever will pry open the world and make it yield up its secrets, even if we have to plunder it and stand it on its head."
G.K. Chesterton observed a similar thing: "But with the appeal to lower spirits comes the horrible notion that the gesture must [be] . . . very low; that it must be a monkey trick of an utterly ugly and unworthy sort. Sooner or later a man deliberately sets himself to do the most disgusting thing he can think of. It is felt that the extreme of evil will extort a sort of attention or answer from the evil powers under the surface of the world."