Pirates raised black flag before red to make surrender credible

A pirate crew boarding a merchant ship by force risked real casualties and damage to the very cargo it wanted intact — a fight was a bad outcome for the pirates too, since every crew member lost or ship damaged in combat cut into the plunder and the crew's own odds of survival. Simply threatening violence in the abstract carried no information a target could act on: a merchant crew with no idea whether resistance would be met with mercy or slaughter had no good basis for deciding whether to fight or surrender.

Pirates also had a reputation problem cutting the other way — a crew known for slaughtering everyone regardless of surrender gave victims no incentive to surrender at all, since a crew facing certain death either way would simply fight to the last. What pirates needed was a way to make a real, credible promise: comply and live, resist and face the alternative — communicated fast enough, at the range of an actual approach, to be believed.

Used a two-flag sequence: black promised mercy for immediate surrender, red signaled no quarter if refused, making the ultimatum credible so targets complied.

A vague threat gives targets no basis to decide surrender is worth it.

A reputation for slaughter regardless of surrender removes any incentive to comply.

The staged black-then-red signal, visible as pirates closed in, made the ultimatum believable.

Consistent enforcement built trust that surrender would be honored, so most crews complied.

Make threats credible by staging them: offer a clear, known consequence for compliance, then a worse one for resistance, and enforce consistently.

The system let most targeted crews choose to surrender under the black flag's terms rather than fight, sparing both sides the casualties and cargo damage a battle would have caused — an outcome that served pirates' actual goal, plunder, better than combat would have.