My 2 1/2 year old son has a 3 inch light brown spot on the base of his spine. when he was younger, that spot was always a dark blue/black.

Benign: A midline lower back discoloration in children is common in Orientals and also seem less often in caucasians. Love him and kiss it better. Not to worry.

Mongolian spot: This is curiously named and quite common. It is harmless though I have seen it mistaken by zealots for child abuse with disastrous consequences. Look at some photos online. Tell Junior it is his lucky spot or something.