In the Potter books, "Dark wizards" exist, but they are simply evil people who use magic to evil ends. "Dark magic" is magic which is generally used for evil; the equivalent in our modern scientific world would be such researches as the development of deadly biological and chemical weapons, or the development of torture techniques, or the development of criminal skills such as forgery and swindling. Good wizards must learn the same principles and techniques when studying "Defense against the Dark Arts", but do so from a morally correct orientation.
Based on his decades of experience dealing with demonic influence and possession on a daily basis, Fr. Gabriel Amorth opts to disagree with your position:
"He noted that the books attempt to make a false distinction between black and white magic, when in fact, the distinction "does not exist, because magic is always a turn to the devil."Do you have some comparable experience upon which you base your opinion?