I believe I read somewhere that mollusks do not produce true black
pigment (at least not in their shells), and that the shells which
look black (such as Conus marmoreus or Cypraea onyx) actually have a
very heavy concentration of brown pigments.  Anyone else familiar
with this idea?
It is true in general that diet can influence pigment deposition,
just as it can influence many other physiological processes.
However, it is not likely that an extreme color variation in one
specimen in a population would be due to "something it ate".  First
of all, in order to influence the whole shell, or a substantial part
of it (such as the whole underside), the animal would have to consume
the mystery substance over an extended period of time, while that part
of the shell was being formed.  And the next obvious question would be
- why weren't other specimens living in the same area eating the same
diet?
Paul M.