I wonder, if HHDL and his predecessor did practice black magic, then why didn't they use it against Dorje Shugden? That proves that HHDL and Dorje Shugden are of the same mind, the mind of a Buddha.
That Dorje Shugden is impervious to black magic actually is an indication of the fact that he is a Buddha; those that would use black magic against him would not really succeed, but that really doesn't prove whether the user of black magic is a Buddha or not.
Black magic by the definition of praciticing with a worldly motivation wouldn't happen from a Buddha of course, since they always have a perfect motivation. The appearance of black magic could be performed by an emanation of a Buddha, though, and we might not really be sure if he is a Buddha or not, thus we wouldn't be sure if it were black magic or not.
For example, on the theory that Milarepa emanated a form that appeared to be not yet awake the lifetime he studied with Marpa, before he met Marpa, Milarepa performed deeds which were seen as non-Buddhst black magic (hailstorms that killed people). However, if Milarepa was already a Buddha emanating to show us how to have perfect devotion to a lama and acheive Buddhahood in one life, then his act of black magic isn't really black magic, because Buddhas do not really do any act without a perfect motivation.