The black tunic that both Mordran and X's uncle wear is normal attire for a male black fairy.   It's not exactly a uniform -- it's more like the equivalent of a suit and tie for men in our world, only a less formal than that.   It's basically what all the male black fairies wear, though the types of sashes, shoes, or accessories vary completely according to the person.   Mordran wears a black sash, which is fairly standard.   X's uncle is more flamboyant, so he wears a colorful one.
 
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