Navy blue comes from the sea. When the Royal Navy issued its first officers' uniform regulations in 1748, it chose dark blue; the color was first called marine blue, and by around 1780 the British press was calling it navy blue.
In lingerie, navy is the blue that behaves like a neutral. It is common in cotton basics, men's-style briefs, pyjamas and striped sleepwear, and it offers an alternative to black in lace for buyers who find black too stark. Navy lace with white or ivory trim has a crisp, nautical look.
Under dark clothes navy is discreet; under white it shows clearly. The classifier's hardest call is navy against black. In dim photos they merge, so the blue in the highlights of satin or in the edges of lace motifs is what separates them.