C Coward writes:

A bit of a follow-up. I've noticed that when the leaves are damp (either
from rain or dew), the discolouration looks darker, almost black. When dry
it is light brown, with a slight sheen. Disturbingly some plants that have
been inside most of the time, but outside recently hardening them off,
also look like they have a similar light brown discolouration. Some of the
time these plants have been near the big, affected ones outside.