? The black magic idea is true in some ways, but I have a different explanation.
Muslims seem to have a big problem with Pagan religions. One of the biggest reasons is that Islam teaches strict monotheism. God is one, there is no trinity or partners or lesser gods to get in the way of this Oneness of God. The Shahada(testimony of faith) reads as "There is no deity but God." It's such a core belief in the religion that a Muslim is supposed to say it at least 17 times a day in prayer.
Now, Paganism is unabashedly polytheist, and has been that way for a loong time. Islam says this has gone back as far as Abraham, whose father built idols and his family worshipped spirits. That just doesn't mix with Islam.
The Qur'an spends a good deal of time saying that polytheism is wrong, that Satan spreads polytheism as a trick to make us forget the real God, and polytheists will be in trouble on Judgement Day. One of the biggest sins you can make is "Shirk," associating partners with God. You don't worship angels or anything on this earth, since God hates it when you give the credit to something other than Him.
Most of the english translations of the text put the word "Pagan" in place of other words like Idolator. Even though Witches seem to be on the rise (a new age comeback?), Islam has been against it from the beginning.
Now, let me point out that I don't have anything against witches, since the ones I've met are nice and decent people, just with what they do. You might as well label this as "Witches and Monotheists", since Christians dislike witches also (let's not forget the burning). Islam takes their dislike a step further, since Muslims believe Christianity has been "corrupted" by witchcraft; moving Christmas to coincide with Roman holidays, mixing Easter with pagan traditions (eggs and bunnies). The Qur'an says not to make the same mistake, and "don't be like the pagans."
Note: I'd reccomend reading Paganism vs. Neopaganism, since it corrects my arguement.
The other reason is that Muslims believe in black magic. There are hadith where a witch cast a spell on Prophet Muhammad(saw) and made him sick for a time, and tried to possess him. Performing black magic is therefore one of the worst sins.