A place where Ancient Ancient armies can peacefully retire to...
Many "Black" paints are really, really, really dark blue. Try adding a teeny bit of yellow to some black and see what happens.
Yes, blacks are very difficult to make - especially recently as the best black dyes were banned because they were carcinogenic! Rifle Green as a colour wasn't standardised until 1890. Before that, a range of dyes were used that faded to a range of colours... so 1897 can justify a slightly 'off' colour by saying the uniforms in use were from old batches of cloth. Or, I can simply go back to my mantra 'toy soldier' and not worry as much about accuracy. As such, I'm trying 975 Military Green - it isn't really 'grey' enough but... see how it looks, paint over if don't like the look.
I end up mixing most of my colours since craft paints are designed for painting soldiers. My rifles end up being painted with a very dark mix of forest green and black.For blue uniforms, pants etc I mix dark blue and black as I can never find a blue as dark as my old uniform. .
Rifles I use a range of browns/brownishes depending what's at hand - usually 983 Flat Earth though. For dark blue uniforms I've pretty much standardised on 899 Dark Prussian Blue. I try not to mix my own colours these days as I have this need to touch up too many mistakes - and it's not easy to match batches.