To dream of black magic represents obtaining your wishes through underhanded tricks. Giving away all your integrity or morality in order to manipulate others. Getting what you want through deception, evil, and treachery.
To dream that you are performing black magic may be a sign that you are too willing to sacrifice all that good in yourself to achieve your goals. Leaving behind your integrity as a means to an end. A willingness to "sellout" for the long-term in order to get short-term power.