How often do you hear people talking about feeling blue or have the blues? An entire genre of music comes from this phrase. Who knew that came from the world of sailing? See-the-sea.org explains the popular phrase comes from a custom that was practised when a ship lost its captain during a voyage. The ship would fly blue flags and have a blue band painted along her hull when she returned to port.
According to dictionary.com this phrase derives from the days when the crew of ocean going ships might be a little less than hygienic, so they needed to present a certificate, carried by a ship, attesting to the presence or absence of infectious diseases among the ship’s crew and at the port from which it has come.