The power of purification!
That's one of the two abilities possessed by this shadow, the Divine Angel.
She's not built to harm others. Her attributes are all healing and purification, against curse attributes and dark elements, her powers are the best counters!
Since the dark mist and black lights could turn normal creatures into grotesque monsters that attacked anything in sight, it's fair to say the black lights are not in the healing and pure category. In this case, using her new shadow purification power was the best choice.
