I found this explanation elsewhere on the net. It is not an "official" answer, but it makes sense.
Back in the day, Carroll Rosenbloom was asked about his redesign idea. When asked about helmet colors he stated, "I would like the blue that matches the jerseys and pants stripes of the athletic gold pants." However, the equipment manager remind him of when Denver tried to do this in 1968. When he saw images of the Denver Broncos helmets, Carroll then stated, "That blue is too bright!" "Looks like the Charger's pacific blue." "Do you have anything else instead?"
The next day, Carroll was sent the helmets that were of navy blue with the athletic gold horns. Carroll was surprised, "This is too dark for the royal blue helmets." "But since there is only two exceptions for the blue helmets, I guess I go with the navy blue instead of the pacific blue that the Denver Broncos use."
Same thing went with the New York Giants. They couldn't find a helmet shade of blue to match the uniforms, so they had to use navy blue instead. Just like the Rams. The only two helmet shades of blue back then were the Denver Broncos azure and the navy.