Another cranky old man chimes in

H.L. Mencken once wrote that "every normal man must be tempted at times to spit upon his hands, hoist the black flag, and begin slitting throats".
For those whose knowledge of things piratical is limited, the black flag (with or without a symbol, often a skull and crossed bones like the one above) denoted the intention to attack, while pirate ships would fly red streamers to indicate that they would 'give no quarter' (meaning to take no prisoners) in the ensuing battle.
In addition, a black flag was the first and only symbol used by Muhammad to represent his religion, and it was the flag flown by Abdur Rahman Khan over his caliphate in Afghanistan from 1880-1901.