Acrocyanosis is persistent blue-ish discolouration of the extremities, most commonly in the hands, feet, nose and ears - sometimes with a cold and sweaty feel to the skin. In the benign form (called Primary Acrocyanosis) which is the most common, there are no other symptoms: and usually no pain, and no loss of function.
It is relatively common, and was first described more than 100 years ago. It is assumed to be related to poor blood circulation associated with spasms in the arterial system. But no formal research has pinpointed a cause. There is currently no effective treatment.