The story
Coined in reference to body in white. * (carbon fibre composite): Blend of black + body in white. From the fact that carbon fibre reinforced composites are typically black in colour, and not whitish (bare wood, bare metal, gray primer coat) * (body shell): From being the skin of the vehicle, opposite to the body in white, the skeleton of the bodycabin, as black is opposite to white.
Etymology from Wiktionary, CC BY-SA 4.0.