Primer Black = check
Blue base coat = Check
Now water down some black paint apply it over all the armor.
This will define the edges & darken recesses & give you that look of depth
( make it more interesting )
From here you can dry brush a mix of blue & a little black - small amount of pigment on the brush -
Add a little more blue to the above Blue & Black mix so it will be a little more light than the first layer.
You can use as many layers of this till you reach a pure blue.
To highlight use a detail brush making reflected light on edges or where you wanna using a mix of blue & lighter blue or maybe bone/ white ect.
Thas the most basic way.
The skulls are best saved for the last stages.
I would suggest medium brown base coat - then wash with a little black watered down
( not too dark ) - let dry - be sure it has enough black around the edges to define it. If there are spots that need coverage touch up with non watered down blk - or rewash till it looks cool.
Using the same medium brown base coat to pick out the overall shapes, highlight with a lighter brown - mixing to bone or off white - with pure white spots.
Keep up the work your not doing that bad
Also dont tell Gamesworkshop but, almost all painters use other brands of paint
( which are much better ) to paint their GW minis
You can save a lot of Time & money by investing in Vallejo Model color paints.
More pigment = Less layers, more details
You can cover areas with just a few coats of smooth watered down paint & if you mess up the first time you can try several times before you start losing details.
TH