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 
      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.