Dermal melanosis

Dermal melanosis are flat blue or blue/grey spots with an irregular shape that commonly appear at birth or soon after. Congenital dermal melanocytosis (formerly called Mongolian blue spots) is a type of birthmark.

      References Optimizing Q-switched lasers for melasma and acquired dermal melanoses 30027914
      The Q-switched Nd:YAG laser is a well-known treatment for pigmented skin spots, both on the surface and deeper layers. Usually, several sessions are required for good results. Using a low-energy Q-switched 1064nm Nd:YAG laser (multi-pass technique and larger spot size) has been proposed as a way to treat melasma.
       Dermal Melanocytosis 32491340 
      NIH
      Congenital dermal melanocytosis, also known as Mongolian spot or slate gray nevus, is one of many frequently encountered newborn pigmented lesions. It is a type of dermal melanocytosis, which presents as gray-blue areas of discoloration from birth or shortly thereafter. Congenital dermal melanocytosis is most commonly located in the lumbar and sacral-gluteal region, followed by shoulders in frequency. They most commonly occur in Asian and Black patients, affect both genders equally, and commonly fade by age 1 to 6 years old. Congenital dermal melanocytoses are usually benign and do not require treatment.