Salutation; refers to the artificial horizon indicator on an aircraft instrument panel. Blue represents sky and black represents the ground. If the black side is up, this will not end well. Similar to truckers' salutation of "Keep it between the paint" and satirical theatrical salutation of "Break a leg."
Said to a spouce: "See ya later, honey; keep the blue side up!"
To a friend: "Remember to keep the blue side up!"