I have had a small blue "mark" beneath the surface of the skin on my left shoulder for as long as I can remember. It does not itch. What could it be?

Blue Nevi. A blue nevus is a rather unusual but non-cancerous mole. The blue nevus is a dark blue colour because the pigment cells (melanocytes) are deeper in the skin than in commoner brown moles and freckles. Characteristically, the view seen with dermoscopy is a uniform steel-blue pattern. Blue nevi may be found anywhere on the body. They usually appears on the skin of older children and teenag.