I mean, if my Cleric of Selune were to use a Dark Justiciar magic armor blessed by Shar, I'd expect some sort of corruption or influence or something to show that Shar was messing with her.
And you wouldn't like to entertain the idea that Shar may allow your Selune cleric to use that armor with no nasty side effects to show your cleric how kind and deserving to be worshipped she may be?
