Since the souls are fused together in this case, Shima would find it mostly impossible to purge the demonic aspect without just ripping the whole soul out.

Also, from what my understanding of black hellfire was, they wouldn't be more affected by it than average, since they aren't more soul than a human is. Or I guess, they have more soul, but in the same sized package. So it would suck for them, but not more than standard. Unless I am misunderstanding how that ability works!