Darkling in this light..." (The Man-Kzin Wars, p. 158)
"...the woods loomed darkling..." (Man-Kzin Wars III, p. 169)
It just means "dark" or "in the dark" and is definitely a Wellsian touch.
On Earth, some species hunt - stalk, pounce, kill and eat; one species thinks and reflects. Thus, we distinguish between hunters and thinkers. Poul Anderson's winged Ythrians and Larry Niven's feline kzinti do both.
"A hunter's wind blew...The wish that it roused, to be yonder, to stalk and pounce and slay and devour, grew in Weoch-Captain until he trembled."
-Poul Anderson, "Inconstant Star" IN Larry Niven, Ed., Man-Kzin Wars III (New York, 1990), pp. 167-310 AT p. 169.
His fur stands up. His claws slide out. His fingers bend. But he walks steadily to meet the kzinti High Admiral. As his partial name and rank indicate, Weoch-Captain is a rational animal able, usually, to control his instincts. Those instincts, when unrestrained, will make him run through the forest and attack his prey with claws and fangs like a tiger. Such a combination of hunter and thinker is truly alien.