Method to basecoat & initial highlight Night Lords "en masse" with an Airbrush. Using a Zenithal undercoat and highlighting approach.
NOTE: Throughout this entire recipe you should focus on doing models in large batches with an Airbrush. Method is intended to give you a whole-model gradient highlight on your Night Lords CSM Infantry for batch painting.
NOTE: If you have not applied a Zenithal Highlight (from Step 2) then Leviadon Blue will be very dark over the black. It will look like a blueish tint to your black undercoat, and it is easy to over-apply. If you have not Zenthial Highlighted then you should be more generous with the Initial Highlight step accordingly as there will be less color variation in your Leviadon Blue.
OPTIONAL: If you have Zenithal Highlighted and then applied Leviadon Blue lightly enough you can stop here if you like the result. If there is enough color variation from you Zenthial Undercoat then your Blue should have some natural highlights. Transparency of Leviadon Blue applied with an airbrush should aid with preserving the Zenithal highlight.
At this point, you should have a set of Night Lords with a whole-model "Gradient" highlight of gradually lighter blue tones. Intention is to then paint the metallics, cloth, weapons, etc. to "break up" the gradient to avoid it looking artificial.
You can also apply a wash of Nuln Oil or Drakenhof Nightshade to smooth out the colors if the gradient is too bright for the taste of your Night Lord.
I would note that all CSM Infantry are well-suited to this technique (from any legion) as CSM have a lot of trim to break up the whole-model highlight. I have had applied this method to 20-30 models in one sitting (CSM Infantry, Havocs, Bikers, and Raptors) to speed up getting my Night Lords table-ready.