Black leaves are not uncommon. Everybody knows the dark-red varieties
of Prunus and Beechwood. The leaves of these plants look quite brown, e.g.
very close to black and the trees do not show any impairement.
In these cases however it seems that leaves are rejecting not just green but
also red,and the mix of green + red is brown. So the trees shouldn't have
a problem of overheating.
There is not clear reason why plants are green and not black, or brown. But
probably the reason is in the phylogenesis. Terrestrial plants originated
from chlorophyta, e.g. from green algae and still are green as those weeds
What do you think ?