Board Thread:Fourth Databook/@comment-4366149-20141122004623/@comment-3279282-20141205210045

Well, it's actually quite easy:
 * Evidence 1: The black flames of BR:K are Amaterasu's flames, as stated in the databook.
 * Evidence 2: Sasuke can use Kagutsuchi without BR to control the flames, as stated in the manga.
 * Also: It's an elemental chakra nature, so there's something released with Blaze Release. Can you release Keitaihenka? No. Can you release black flames? Yes. According to E1, the black flames are Amaterasu's flames. Since the black flames are released through Blaze Release and are the flames of Amaterasu, it means that Amaterasu - which used the black flames - is Blaze Release.

But I can imagine NoJutsu saying that there's no proof that the black flames are what is released with Blaze Release, because there's nothing that suggests it. The fact that Sasuke used Kagutsuchi on already released black flames doesn't count, of course.