if you watch when barbossa is talking to elisabeth while she eats she gets up and stabs him with the knife and he pulls it out and says now after killing what do you plan on doing if you look at the knife it has blood on it
George, that is not what he is pointing out. He is saying that the coins must be returned and each cursed person's blood needs to be put in the chest. This is why both the blood of Wiliam and Jack are needed -- and why Jack cuts his own hand and has his blood on the coin he tosses Will, and why you see Will drop two bloody coins in the chest. I think this is therefore a good question.
However, I have a proposed answer, if a bit contrived. When the pirates realized they were cursed they may have each cut a little piece of their body (just a cut of flesh from somewhere, or perhaps a pinky toe) and added it to the chest. That way when all the coins are in the chest, those dead parts would turn live and bleed, fulfilling the requirement for blood. Why would they do this ahead of time? In case one of the undead pirates is not around when the coins are all returned to put in blood then (fell overboard, etc). Perhaps a stretch.
It may also be that it is not blood that is needed, other cellular material might meet the requirements of the sacrifice.