I think your giving the ability more credit than it deserves, doing something like making wings out of blood would drain his own blood supply and kill him. and who knows if they can actually rip the blood out of a living body like they do with plants... besides, its clearly a high-end master level ability that can also only be used with the power boost from the full moon.
One things that concerns me though *spoiler alert*
Hana said it herself that it took her years to master the technique, but here we see Katara performing it moments after learning of its very existence. At least when she became a master in between Waterbending Master and Seige of teh North we had the sastisfaction of knowing there was indeed a time skip between the episodes. In this case though she hears about the technique, and can instantly preform it on its very master. thats a little much I think.
Edited on 10/25/2007 7:19pm