I would suggest that Vallejo Dark Prussian blue is if anything too light , and too vivid to represent Indigo dyed uniforms. All dark blue uniforms, at least until near the end of the 19c , were dyed with Indigo, which creates a very dark purplish blue, often a sort of blue-black. It gets dusty and dirty , but scarcely fades.
Might I suggest adding a little Flat Red to the Dk. Prussian Blue, which should get you to a more indigo shade.
Incidentally , the artificial pigment Prussian Blue, has ABSOLUTELY NOTHING to do with the dyeing of uniforms, Prussian or otherwise. I was first synthesised as an artificial pigment ( NOT dyestuff) in about 1715 in Prussia, hence the name. It's incredibly vivid, if undiluted by other pigments, but unfortunately it's widely used in model paints, where it creates an intense , rather unrealistic effect.
