A blue naevus is a specific type of skin blemish characterized by a dark blue or black hue and a well-defined border. These naevi are benign, meaning they are noncancerous. They consist of a cluster of melanocytes, which are cells responsible for producing pigment. The accumulation of these pigment-producing cells gives rise to the distinctive appearance of blue naevi. While typically harmless, any changes in size, shape, or color should be monitored by a healthcare professional to ensure they remain noncancerous and do not develop any concerning features.