From Creatures Wikia
The Dusky Grendels were created by Elf. They are dark in color. The males have blue hair, while the females have purple. They are much less aggressive towards norns than most grendels, and are generally afraid of them. Actually they are pretty much afraid of everything. Leave them alone for 5 minutes and you'll come back to "shadow intensly scared." Tickle them to comfort them.
Dusky Grendels also have no mating instinct. Their genetics tell them that pushing norns decreases sex drive. To compensate, they live longer and have a better immune system than usual, as their genome is actually derived from that of a Bruin Norn.