Corid would not harm them if you choose to use it, but I still think there could have been some picking of the vent, either from a prolapse that went back in on it's own, or from one of the hens being aggressive. I have found blood in the coop before, and not been able to find where it came from. Combs get pecked and bleed pretty easy, getting spread all over others. Also feather-legged breeds can get blood feathers pecked, and cause a bloody mess.