Not Templars, but my Crimson Tears are largely black and white:
I prime black, then go over it with a couple of coats of Army Painter Matt Black. Highlight with Vallejo Model Colour Neutral Grey.
The white is built up over the black starting with Vallejo Ghosty Grey, moving onto
GW Ceramite White and finishing with VMC White. Then I go and do shading either by using VGC Stonewall Grey and building back up tp pure white or Vallejo Pale Grey Wash
Regarding the points in your
OP
Washing black with black will achieve little. As above, you could start with a very dark grey (Vallejo Game Colour Charcoal Grey is definitely the darkest I've encountered) and washing that heavily will probably get you a near black and almost pure black on the recesses.
If you paint white over pure codex grey, you'll need a lot more than a coat or two unless you're slopping it on to detail obscuring extent.
I assemble mine in pieces which makes doing the shoulder pads a lot easier. and generally do the white first.