User blog comment:Xilinoc/Everything but the Rain op. 4: Dark of the Bleeding Moon/@comment-5198194-20130328001941/@comment-5695542-20130329224339

The Getsuga Tenshou has always reminded me of fire for some reason (not saying it is or Isshins zanpukto is flame type) not fire in like we know or Yamamoto's flame but a type of fire.

On the topic of that stuff coming from Isshin's blade he was in the same pose and the "reiatsu" encicleing him was similar to when Icghio charged/began to fire a Getsuga Tenshou so it could he was getting really to fire a Getsuga Tenshou. Or are you talking about when Isshin released Engetsu?