User blog comment:IamJakuhoRaikoben/Sorcerer Magazine, Issue 27: March 2014/@comment-4829046-20140404155313/@comment-4829046-20140404165803

Or it was just following Bleach principles. When Shinigami lose power(Rukia) the black hakama is gone so they only have the white underbit. But then again.... that was back when Bleach actually tried to made sense.