The Collaborative International Dictionary of English v.0.48:

Navy blue \Na"vy blue`\, n.
   1. A dark grayish blue. Also called navy.
      [PJC]

   2. Prussian blue. [archaic]
      [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

navy blue
    n 1: a dark shade of blue [syn: dark blue, navy, navy
         blue]