Despite its name, the blue monkey is not noticeably blue: it has little hair on its face, and this does sometimes give a blue appearance, but it never has the vivid blue appearance of a mandrill, for example. It is mainly olive or grey apart from the face, the blackish cap, feet and front legs, and the mantle, which is brown, olive or grey depending on the subspecies. Typical sizes range from 50 to 65 cm in length, (not including the tail, which is almost as long as the rest of the animal), with females weighing a little over 4 kg and males up to 8 kg.
The blue monkeys live in social systems where females stay in their natal groups while males disperse once they reach adulthood. As a result, blue monkey groups usually consist of one male with several females and infants, giving rise to matrilinear societies.