The black magician and the black horse are half-evil elements whose relativity with respect to good is hinted at in the exchange of garments. The two magicians are, indeed, two aspects of the wise old man.
Jung argues that the black magician is not simply evil but represents one of two complementary aspects of the archetype of spirit, its dark pole relativized against the white and united in the figure of the wise old man.
, The Archetypes and the Collective Unconscious, 1959thesis