A blue nevus is a type of skin lesion that appears as a blue or blue-gray spot on the skin. It is formed by a collection of melanocytes, the cells responsible for producing pigment. These lesions are usually benign, meaning they are not cancerous, and can occur anywhere on the body, though they are most commonly found on the back, scalp, and face. Blue nevi can vary in size and shape, and they may be present at birth or develop later in life. While they are generally harmless, it is important to monitor any changes in size, color, or shape, as this can indicate a need for further evaluation by a dermatologist.