melanism
Concise Oxford English Dictionary © 2008 Oxford University Press:
melanism /ˈmɛlənɪz(ə)m/
▶noun chiefly Zoology darkening of body tissues due to excessive production of melanin, especially as a form of colour variation in animals.
– derivatives
melanic /mɪˈlanɪk/ adjective.
melanic /mɪˈlanɪk/ adjective.
'melanism' also found in these Oxford entries:

