User blog comment:CNBA3/Tartarus' Army!/@comment-4829046-20131104025214/@comment-4829046-20131104031020

That's just the thing, Kyouka just said "Those who are weak and are not worthy of this magic will end up that way(black shikigami)" but the thing is that strength =/= skill =/= knowledge

Even if you are strong in power doesn't mean you are skilled per se. Hibiki has knowledge and tactical ability and perhaps even skill cause he was able to do so many things so quickly and efficiently but he has low attack power and defense so I don't think he can gain the power up, his body won't be able to handle it. So this "worthy" aspect is a bit iffy since its not stated what it truly means to be "worthy" of the magic.