Black magic is used for evil purposes by inovking the power of the devil. God's son Jesus was snt here to 'deliver us from evil' and the devil, or satan, who is customarily thought to be the embodiment of evil. According to the bible, he infuses his way into people's thoughts, decisions, & responses to situations. He is at the core of addiction, immoral sex, destruction of family, etc. He is the antithesis of God and Christ. Many Christians believe themselves to be engaged in something termed "spiritual warfare" (kind of an oxymoron huh?) which denotes activities believed to be directly challenging evil supernatural entities under satanic influence. Since the practice of black magic is diametrically opposed to the tenets and beliefs of Christianity, my answer to the question 'Does Chritianity forbid the practice of black magic' is yes.