With regards to your questions, bluish skin tone is known as cyanosis, which is caused by a lack of oxygenated blood to the specific area.
In your case, Central Cyanosis (Cyanosis
of Centralized Systems like mouth, tongue or the core
of the body) can be caused by many issues, generally relating to the Central Nervous System, Respiratory System, Cardiac Disorders, Blood Disorders and/or other issues (Altitude, Hypothermia, Obstructive Sleep Apnea).