Board Thread:Lore Discussion/@comment-13199219-20130809220651/@comment-4092092-20130815035103

to get back on topic; perhaps mortals having Black Souls are a sign that we're so much evil but rather the black soul gems that can hold our souls are evil.

They do after all come from the daedra, and seem to be illegal for the most part (few legally mages sell them, compared to other soul gems).