A really, really dark blue or you could call black with a blue hue.
black and blue and silver
A darker navy blue
you don't
If you are trying to make black paint with blue paint just add an earthy colour (brown) :)
Floquil and Scalecoat make the G.N. green. The Big Sky Blue paint scheme was blue, black, and white, so I'm not sure what color you are asking about.
Blue + black.
Blue & Green make turquoise
mix brown and blue paint
Use black paint on its own, otherwise you will need to keep adding black until the grey becomes black enough. Adding a very dark blue to grey will make a black, but it won't be a proper black, more like a bluey black.
Blue And Yellow
Mixing blue and black will make a very dark blue.
Green