this cannot be done the easy way, neither in jass nor GUI. because you cannot simply change the coloring of a special effect.
and even if, setting the tinting color of the lightning models to 0,0,0 will make them completely invisible instead of black.
i'd say you need somebody to make you a black lightning custom model for this effect.
If you weren't aware of the site The Hive Workshop, it has a large selection of custom models, including a black/purple monsoon strike.
[edit] Too late.
yeah! that link is gold, where did you find it? couldnt somebody sticky this in the forum?