A fabulous king of Athens who gave name to the Ægean Sea. His son, Theseus, went to Crete to deliver Athens from the tribute exacted by Minos. Theseus said, if he succeeded he would hoist a white sail on his home-voyage, as a signal of his safety. This he neglected to do; and Ægeus, who watched the ship from a rock, thinking his son had perished, threw himself into the sea.
This incident has been copied in the tale of Sir Tristram and Ysolde. Sir Tristram being severely wounded in Brittany, sent for Ysolde to come and see him before he died. He told his messenger, if Ysolde consented to come to hoist a white flag. Sir Tristram's wife told him the ship was in sight with a black flag at the helm,
whereupon Sir Tristram bowed his head and died.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894