some fuqahaa have said that doing sihr (black magic) is riddah from Islam, in and of itself. some others have said that it is a major haram and not riddah unless someone believes it to be permissible. i follow the opinion that it is riddah in and of itself. regardless of the generic ruling in itself, at a practical ground reality level, black magic is done by the aid of shayateen, and the magician simply can't get access to their powers unless and until he commits open blasphemies like disrespecting the Quran, bowing to idols and fire etc., wal 3eyadhu billahi ta3ala and no one disputes that such acts are kufr and open riddah. Muslims these days need to be taught about all manifestations of kufr - lafzi, fe3li, and i3teqadi. Allah and His Messenger know best.