Roguish yet charming Captain Jack Sparrow's idyllic pirate life capsized after his former first mate, the wily Captain Barbossa mutinied and commandeered Jack's ship, the Black Pearl. Barbossa marooned Jack on a deserted island, but Jack escaped. Some years later, Jack arrives in Port Royal to steal a ship. His plan is sidetracked when he saves the governor's daughter, Elizabeth from drowning. Because he's a pirate, he's promptly jailed and condemned to hang. Barbossa attacks Port Royal, searching for the last gold coin needed to end an ancient Aztec curse. Barbossa kidnaps Elizabeth, who has come into possession of the coin. He mistakenly believes she's the child of long-gone shipmate, Bootstrap Bill. Will Turner, the town blacksmith and a master swordsman, loves Elizabeth, and he enlists Jack's help to rescue her. Jack agrees, but neglects to mention the curse that has doomed Barbossa and the other pirates to an immortal life as ‘living dead.' (Jack escaped the curse.) After busting Jack out of jail, the two hijack a British naval ship, collect a motley crew, and chase the Black Pearl to Isla de la Muerta, the pirates' island hideout. Along the way, Jack tells an incredulous Will that Will's father is actually the pirate known as Bootstrap Bill Turner. Commodore Norrington, who also loves Elizabeth, is in hot pursuit of Jack and Will.
Answer: The curse needs the blood of everybody who took a coin from the chest. All the other pirates have already contributed so, as the movie opens, the only blood needed is Will's, substituting for his father. During the finale of the movie, Jack takes a coin from the chest, adding himself to the curse, so his blood is now required as well as Will's.
Tailkinker ★
But I didn't see any blood on the coins, and none of the pirates cut themselves, even before Will became part of the mix.
Yes, the other pirates did cut themselves before Will came into it, off-screen. The lack of blood on the coins can simply be explained as most of it dripping to the bottom of the chest, it being washed away by storms blown into the cave, or by the fact that they didn't drop that much blood on it in the first place.
When they had Elizabeth they believed she was Bill Turner's daughter, but they all thought the curse had failed, none of them had cut themselves so it makes zero sense.
They had been collecting back the coins for years. During that time they repaid their own blood. All they needed was the last coin and the blood of Bill Turner to break the spell.
lionhead