My 8 months son was born with flat bluish birthmark on hip and leg. now, i noticed 2 new spots same color and texture. is it important?

Maybe: There are several important neurocutaneous syndromes which present with birthmarks. You have to actually see them. Go see your doc or a geneticist right away.

Doubt it: The bluish or dark birthmarks that occur on the back or posterior aspect of arms/legs/trunk are usually based om excessive melanin production in patches of skin. Often called Mongolian patches these are benign. Your doc can confirm or exclude their cause.