Royal blue might have been common before, but navy blue is the new black now. I think the only teams that still use royal blue (excepting the Bills) are the Colts and the Giants.
So only 10% of the league? And since we're not talking about which *current* uniform it looks like, the current status is pretty much irrelevant anyway.
But since you brought it up, what teams use navy blue currently? The Bills actually switched AWAY from it with their new unis. (I'm having to look this up, of course):
Seahawks: "Seahawks blue", with Navy trim and pants.
Bears: "Dark Navy". I'd actually forgotten about them, despite ALWAYS having been Navy.
Cowboys: Navy. Switched from Royal to "Dark Blue" for the '97 season and then to Navy in '02.
Broncos: "Broncos Navy". Switched away from Royal for the '97 season.
Lions: "Honolulu Blue". "It's the kind of blue, I am told, that will match with silver." (Lions VP and GM at the time, who chose the color.)
Texans: "Deep Steel Blue". Not quite Navy, but close to my untrained eye.
Chargers: Navy and "Powder Blue". Added their Powder Blue's in '07.
Colts: Royal Blue.
Patriots: "Nautical Blue" (since 2000). They couldn't be bothered to just call it Navy? Really?
Giants: "Dark Blue". Looks pretty much like Royal Blue to me.
Titans: Navy and "Titans Blue".
The conclusion of this little excursion: Navy dominates and has for a long time. There was a switch away from Royal Blue about 10-15 years ago (which matches up well with what you said).
The Lions' Honolulu Blue looks a lot like the Powder Blue of the Chargers... which is pretty much just baby blue, IMHO
. The same with "Titans Blue".
The Seahawks, who used to be royal blue, are going with a new uniform design in 2012, coinciding with the switch to Nike. Hopefully, they'll at least get rid of the nauseating green.
Another completely irrelevant historical tidbit: The Colts actually had Navy and White uniforms for a couple of years way back in the 50s.