Cobalt Blue Deep shades to a LIGHTER neutral shadow color by adding it's opposite color Orange. Since Cadmium Orange and Cobalt Blue are opaque it will not fully darken to black but will make a beautiful neutral gray.

Here is a painting by Winslow Homer, He is using the oppositions Cobalt Blue and opaque orange. Also Ultramarine Blue and Burnt Umber that will make a very dark neutral.
He seemed to use both here.
It takes two transparent oppositions to mix a neutral dark, or one transparent and one translucent, two translucent colors or one transparent and one opaque. Two opaque colors just won't do it fully dark even though it will be neutral.

True Cobalt Blue is a rare pigment today.
    The artificial colors are in this Opaque Azure range,
there are no Transparent Azure Cobalt's available. :(
The Cobalt Blue color has a little less Magenta in it than the color of Ult. Blue has.

Cobalt Blue Light Opaque.

Mixed with this opposite color.

Cadmium Orange is a dye on a white base.
As so it is opaque and can't be mixed real dark, but it does make a beautiful neutral gray.
Cadmium Orange Opaque

Mix Burnt Sienna with Cobalt or Ult. Blue together to make a Burnt Umber color
Burnt Sienna Translucent

Burnt Umber Translucent

 

Here is a crystal in nature that matches the color path I am talking about.

VESUVIANITE, Ca10Mg2Al4[SiO4]5[Si207]2[OH]4, H7, SG-3.3,

MAGNESIUM as an element in this crystal will go dark and opaque or dark and transparent, either way it does it with flair.

ALUMINUM will make yellow, transparent, standard #1[YYYY], and blue, transparent, standard #7 [MMCC], centering to neutral.
These crystals in the tetragonal system with some masses in the orange or azure range.
By centering yellow to brown, the three analogous colors of this gem, yellow-green, yellow and orange go to brown.
On the other side of this analogous range is blue, Ultramarine Blue Hue crystals were found in Norway.
 

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