A while back I posted several brief tutorials for painting my 28mm ACW armies, and I shared the specific paints that I used.
Just looking at my Flickr page, I came across a photo from another painting project that I think really helps show the deviation from painting Union forces with "dark blue" and "light blue."
When painting my Union forces, I kept in mind and had at hand a number of Don Troiani images and sought to match up the shades of blue he achieves in his paintings with my miniatures. And in doing so also attempt to help make my Union armies in any scale "pop" as much as possible on the tabletop.
And the two best colors I found to achieve this are two GW Contrast paints, "Stormfiend" and "Nighthaunt Gloom":
At the risk of sounding like I'm saying or claiming there is an absolute "right" or "wrong" color for something so broad as Union blouses, coats, and trousers, there is of course no such thing. I can easily find photos of original Union uniforms that are indeed quite a dark shade of blue. As well as trousers that are indeed on the lighter side of "sky blue."
However, I did base my paining on the general assertion that MUCH of the Union uniforms issued were indeed not the extreme dark or light blue, but indeed were closer to "medium" blues along the spectrum. These medium blues were either the uniform's color at the time of issue, or they became faded from use.
Another aspect of "Union blues" that I sought to achieve was Troiani's observation that many Union coats faded to either a sort of greenish blue or grayish blue. And that Union trousers after some fading and after being washed took on a slight greenish cast that mixed in with the blue.
I also took the knowledge that in many contractor factories, there was often not specifically different colors of blue dyes for the coats and trousers, both were in fact dyed using the same exact blue color dye except for the fact that when the trouser cloth was being dyed, it was a diluted version of the blue used for the coats, making it lighter but nevertheless from the same base color.
Therefore, Union coats and trousers often were actually deviated in color from what is considered navy blue or sky blue.
Troiani shows this extensively throughout many of his works:
And some original coats, trousers, and caps:
And then finally my Union forces painted with the two Contrast colors "Stormfiend" and "Nighthaunt Gloom":
And then finally, my 3D printed Thorin Oakenshield, whose hooded cloak and tunic are painted with the exact same two Contrast colors as all my Union coats and trousers are. You can see from these photos, these colors are anything but extreme "dark" and "light" or extreme "sky" blues. Stormfiend is indeed quite a medium blue with a grayish cast, and Nighthaunt Gloom is a grayish blue with a slight green cast to it.