I'm an ex-airbrusher/custom painter.
I don't mean scale models (although I have painted a few),
I mean full size Bikes, Custom Cars, Murals, Helmets, Clothing, etc
There are many paints that require and/or work best over a black base.
Black primer is common in some automotive work.
Just like using a base white, or white background to make colours more brilliant, black can have interesting effects.
Opaque or Transparent, the base or background will still show through and can influence the top colours.
Black does not reflect like white or silver base, creating a whole new look with some transparents.
I've used black on scale model, both under and on top of the paint colour, to darken seams, lines, cracks.
It collects in and around those areas more than anywhere else.
End of my tutorial...now back to the scale model experts.