Oh no!! How scary! Wonder if he scraped an area where a little vessel was close to the surface or something. It sounds like you are doing all you can, but how scary to see your baby bleeding like that! I imagine it probably hurts some but I'd bet he's picking at it more because of the corn starch and stuff? I'm not very experienced with bird injuries as the only one I've delt with was a broken blood feather on Twi awhile back... that I pulled off with tweezers and it was fine then.

If he wont' leave it alone, maybe call the vet and see if there is something you can get that will make it so he can't get at it? Hope he heals quickly! Popcorn don't do silly things like that, you scare your mommy!