User blog comment:RoyalGuard-Elite/Isshin's Shikai/@comment-5551986-20131016195053

He probably has just mastered his Zanpakuto techniques, cladding his sword in Getsuga itself, much like Ichigo does when he attacked Ulquiorra and as for "Burn" when people get hit by Getsuga they occasionally look charred. If Engetsu is similar to Zangetsu, it likes to protect its master, spitting your own blood on it could enrage it, thus explaining the big power-up from his own blood. Anyway, with nearly a anything firey thus far, the artists theme seems to feature flickers of flame.