Look up chromhidrosis. It can be several colors (yellow, green, blue, black, brown -- not just blue.) It's a reaction of sweat and the skin and substances in or on either the sweat or the skin. There are three kinds (of chromhidrosis) -- if my memory serves, each is caused by something slightly different. It's medically harmless. (Except that it's shocking or embarrassing to people, of course.) Talk to a dermatologist if your doctor is clueless.