Navy blue is a very dark shade of the color blue. Navy blue got its name from the dark blue (contrasted with white) worn by officers in the British Royal Navy since 1748 and subsequently adopted by other navies around the world.
When this color name, taken from the usual color of the uniforms of sailors, originally came into use in the early 19th century, it was initially called marine blue, but the name of the color soon changed to navy blue.
An early use of navy blue as a color name in English was in 1840 though the Oxford English Dictionary has a citation from 1813.
In practice, actual blue uniforms of the United States Navy and other navies have become outright black in color, because of difficulties in exact matching of shades of blue from various suppliers.