Dermal melanocytosis

This birthmark is a blue-grey patch of skin most commonly found on the back or buttocks (although can appear in other places too) 

It is completely asymptomatic and does not require treatment. The patches normally resolve by the age of 4 or 5 years. The larger lesions can persist for longer.

References

https://www.stemlynsblog.org/dont-be-rash-petechiae-in-well-kids-at-st-emlyns/https://www.dermatologyadvisor.com/home/decision-support-in-medicine/dermatology/dermal-melanocytosis-previously-known-as-mongolian-spot/

https://dermnetnz.org/topics/lumbosacral-dermal-melanocytosis/