I am not an expert but I have seen more than my fair share of Civil War uniforms. I am pretty certain that is a Union uniform not only for the color, but the general style as well. However, I can’t locate any good pictures to back it up conclusively.
When you’re editing copy or images and you need to make a mark that won’t show up in photoreproduction, you can use a non-photo blue pen or pencil. In the same way as red will show up as near-black in photocopies or B&W photography, a very pale blue - almost robin’s egg, but even lighter - pigment will become invisible to the camera. If you press down too hard and leave a very dense mark with this blue, it will show up.
That’s a far cry from navy blue.
Perhaps over-exposed or faded, but it wouldn’t matter in that case if it were navy blue or black. In this picture for example, his chest area is lighter but his pants were the same color.
Like I said, if the cheap film is overly sensitive to blue light, navy blue may come out lighter than on normal film. However, navy blue – if it’s the same hue we’re thinking of – is fairly dark to begin with, sometimes indistinguishable from black. If that’s the case, there’s not enough going to be nearly enough blue light energy eminating from it to make it show up as nearly white on film. The only case where I can see this possible is if the film were sensitive to only blue light, but even in that case, we’d blow everything else in the scene way out of exposure.
This is what I think of when I think navy blue.
Now, if your navy blue and dress materials reflect much more light than that, I suppose it may be possible. I would have to see the picture to see what the effect looks like.
She would refer to a blue about a shade darker than the banner on this site, or the collar of that shirt as a shade of navy blue. Maybe that is it?
Basically, when taking a photograph you are focusing an image on the film plane and allowing a certain amount of light to reach the film emulsion. The emulsion is sensitive to light. The brightest parts of the image cause the most reaction in the silver halide particles, causing the image there to be more dense and areas of lesser exposure will be less dense. So a white area on the film will appear black, and a dark (black, or dark blue, for example) will appear white. When the film is developed you have a photgraphic negative. When you print the photo, the same thing happens; only the light bits of the negative become light in the print and the dark bits become light. This is your positive image. So if the dress is dark, then its image in the negative will be light and the image in the positive that is made from the negative will be dark like the original.
Johnny L.A. I know that much at least. (I take decent color pictures, but do not claim any skill with black and white photography. My pictures come out well because I can compose, that is all.) I have actually developed film as part of a photography class I took in middle school, which used black and white film. I do not recall whether or not the teacher covered photographing blues (especially dark ones) or not, since it was very basic stuff using rather outdated equipment. (We actually had to learn how to unwind the film from the cameras and not let it get exposed for example, this was in the mid '80s.) It was about that time that my mom showed me the picture of herself as a teenager though. She seemed to be in a sunny spot, if that helps? 
(Though her features were clear, I think other details weren’t as sharp. It was a full length picture.)
Er, I didn’t phrase that too well. I mean we had to manually wind the film up and extricate it from the camera without letting the light hit it, thus ruining the film. :o I actually liked working in the darkroom. I did get an A in the class, but as I recall, we were only graded on one picture, the rest was either various quizzes, or having us do various chores and him grading us on whether or not we did as we should do, or were completely lost. (He went easy on nervous fumbles, and would spend time to try to show the student the knack of things.)
What you’re looking at is a very old, faded, grayscale image, and you’ll never resolve anything by guessing about the colors.
The most distinctive clue is the hat, followed by the epaulets and the buttons.
And by the way, navy blue is an extremely dark color; in some lights it’s indistinguishable from black. There’s no way for it to show up white in a photo, except maybe a highlight on a shiny surface. That’s assuming all the other colors are true, and the photo isn’t extremely faded in general.
Emphasis added.
No, I certainly don’t know the answer, and therefore no springing will take place.
Also, to answer another poster who thought this guy looked familiar, no, I don’t know who this soldier is.
It may be a distant relative, because I received this from my parents. As far as I know, none of my distant relatives hail from the southern U.S. (not that there’s anything wrong with that! 
)
It doesn’t matter. You were certainly using panchromatic film, and all that matters to the film is the apparent brightness of the object, i.e. how much light it is reflecting back. Dark shades of any color turn up as dark shades of grey. Light shades of color turn up as light shades of grey. (There will be some minor variation depending on the spectral sensitivity of the BW film you’re using, but not anywhere near enough for the effect you’re describing.)
This is what I was thinking as well.
Perhaps even a military-school uniform?
Errrr can’t link directly to it but its the junior officer’s frock coat.
The hat looks like the stereotypical Union Army enlisted men’s version. Yes, the Confederates had a variety of uniforms, but I’ve never seen one like that in a Rebel photo.
I think I’ll go with … UNION!
Thanks, everyone, for your expert advice.