THey used a regular aura. I believe the dark aura first came up in the Gamecube game or something along that line. And I think it made it's way in an episode or two when Ash got possessed by a long past soul of sorts. My fic hasn't discussed it yet, but it will have it's time.
I really do have to work on my fic at some point. I might have time all next week, as that week I have off from school.