M-1723 was the original code for a very dark Dearborn blue. If you google it then you may find newer names or codes on the HAMB or Jalopy Journal.
I tend to look a the color chips from my current paint product that I use as long as I know the original shade. There will be several shades in the dark category. It reminds me of the color originally used on the Vought F4-U Corsair fighter planes. It's close to Navy blue. One of my old 51 Mercs is Banning Blue on the lower color but it just a tad lighter than Dearborn Blue. Midnight Blue is close.