When hemoglobin is saturated with oxygen, it is a bright
pink. When the oxygen in arterial blood has given up its
oxygen in passing through the capillaries, and enters the
veins, it is a dark red. A human deprived of adequate ventilation of lungs no longer has bright pink blood in capillaries of his skin, turns a gray blue color, which is
called cyanosis.
Some poisons such as cyanide have same effect. I used to see skidrow alcoholics who were blue because they were taking BromoSeltzer which had both antacid, and acetanilid,
which was very effective against headache, but caused methemoglobinemia, meaning it interfered with O2 uptake by hemoglobin. Now acetanilide has been changed to acetaminophen, as in Tylenol. No more blue faced drunks.