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

I really hope Kubo doesn't leave us hanging on what the stuff coming from Isshin's blade is. I hope it is reiatsu, because that would probably be the best explanation for why Ichigo has Getsuga Tenshou, in my opinion. With Zangetsu having such a similar name to Engetsu I would think they would have a similarity of some sort. "Burn" could be named after the fact that the "reiatsu" flickers and spews upward in a manner similar to fire.