To define cyanosis is pretty much to declare that something is bluish in color. When a person literally starts to turn blue it is also more than just a little bit serious.
I learned this the hard way when my grandmother had a heart attack. When we found her, she literally had a bluish tint to her face and body. It was horribly frightening, and not something that I wish to ever see firsthand again. Something bad was obviously going on.
I’m not sure why people turn blue when they lose a significant amount of air, but they really do.