I think black was like, "We just want your shit and we'll let you live if you surrender" while red was like, "We're gonna kill and then take your shit or take your shit and then kill you, either way you will not survive," kinda thing.
What was the point of the red flags? Or... why would anyone choose to fly a red flag? I'm trying to understand why anyone would broadcast "if we come near you, you're fucked". It seems a bad hunting strategy.
Also, why didn't merchant ships fly red flags to be less bothered by pirates?
Red meant that no quarter would be given, they're going to fucking kill you. Black flag meant they would take your shit without slaughtering you. Granted, fighting back would still get you killed either way. Pirates tended to make examples sometimes, or want revenge, thus slaughter. Of course if they didn't kill everyone, they might press-gang you into service, and now your a pirate too. So red means you're dead.