These are called Birth marks or Mongolian spots usually they disappear by 1 yr of age They are harmless Nothing to worry It will go by itself

As it is the first time and at a single site, so just wait and watch. This feels like traumatic. But as per you there is no history of trauma, kindly monitor its size and appearance of such blue lesions on other site. If it occurs on other sites, kindly visit your nearby pediatrician so coagulation disorders can be ruled out.

Mongolian spots