Theo
If you do use the Derivan blue the following may help:
Over the weekend I was doing some more French Line and decided to define the mix for the dark blue. As mentioned I'm using Derivan French Blue and Vallejo black for this.
Out of the bottle the blue looks thick, deep and glossy, thin this as usual and spread it out a bit on a white palette, this will make it look much more blue. I mix in black until the blue turns to a colour where you're not sure if its really going to come out blue or black – your kinda unsure of it– that's right, that's the 'wet' colour you're aiming for! Use neat blue for mid tone and add white to the mix for highlight.