User blog comment:B214/Question and Answers 6/@comment-134.115.2.117-20161121061708/@comment-9595671-20161121073613

I think that's because in the anime, it has the aura of rias' magic circle embdedd into it, that's why sometimes it's black with red around it or a black/red mix.

So in the light novel, it's likely that she doesn't need to do this just as Riser's regeneration is a passive ability that doesn't require his magic, or maybe the anime just makes it part magic where in the light novel it's not.