Magic comes from "mag," priest. Black magic is the science of the impure priesthood, or those who awaken the consciousness within the ego (pride, anger, lust, etc). See also Goetia.
"Black Magic appeals to the mass mind. It appeals to the principles of our civilization. It offers something for nothing. As long as there is cupidity in the human heart, it will remain as a menace to the honesty and integrity of our race." - Manly P. Hall, from Magic: a Treatise on Esoteric Ethics
"The black magicians have their mysticism, and they always firmly believe that they walk on the good path. No black magician believes that he walks on the evil path. The path of black magic is a broad way filled with vices and pleasures." - Samael Aun Weor, The Revolution of Beelzebub
"Multitudes of schools of black magic exist, many of them with very venerable traditions that teach Sexual Magic with the spilling of semen. They have very beautiful theories that attract and captivate, and if the student falls in that seductive and delicious deceit, he becomes a black magician. Those black schools affirm to the four winds that they are white and that is why ignorant ones fall. Moreover, those schools talk of beauty, love, charity, wisdom, etc., etc. Naturally, in those circumstances the ignorant disciple attains the belief with firmness that such institutions are not evil and perverse. Remember good disciple, that the Abyss is full of sincerely mistaken ones and people of very good intentions..." - Samael Aun Weor, Tarot and Kabbalah
"Evilness is so fine in the world of the mind... Evilness is so delicate and subtle in the plane of cosmic understanding that in reality a lot of intuition is needed in order not to be cheated by the demons of the Mental World." - Samael Aun Weor, Treatise of Sexual Alchemy
"The intellect as the negative function of the mind is demoniacal. Everyone that enters into these studies, the first thing that they want is to dominate the mind of others. This is pure and legitimate black magic. No one has the right to violate the free will of others. No one has the right to exercise coaction upon the mind of others because this is black magic. The ones that are guilty of this grave error are all of those mistaken authors that are everywhere. All of those books of hypnotism, magnetism and suggestion are books of black magic. Whosoever does not know how to respect the free will of others is a black magician; those who perform mental works in order to violently dominate the mind of others convert themselves into perverse demons. These people separate themselves from the Innermost and they crumble into the Abyss." - Samael Aun Weor, Tarot and Kabbalah
"Black Magic is not a fundamental art; it is the misuse of an art. Therefore it has no symbols of its own." - Manly P. Hall