Pre-shading black models

You'll have to lighten the black with some grey or brown paint in various panels and around curves and edges. Its called post shading. Then try a light brown or gray wash.

you may consider putting a "lighter" dark dark gray in the middle of the panels so that there is some small color variation and the panel lines would show by being even darker

Personally, my feeling is that black painted subjects do not show panel lines very well compared to light colored subjects, so I don't bother with enhanced panel lines, except maybe paint chipping around edges of removable panels.

Only thing I can think of is trying to change the way the black reflects the light but trying silver under the black. The problem you will have is a good coat of black will cover everything under it. May not be worth the time or hastle but if that works it could look good (and I claim the idea).

Phil