The blood of one who breathes improperly is, of course, of a bluish, dark color, lacking the rich redness of pure arterial blood.

This often shows itself in a poor complexion.

Proper breathing, and a consequent good circulation, results in a clear, bright complexion.
A little reflection will show the vital importance of correct breathing.

If the blood is not fully purified by the regenerative process of the lungs, it returns to the arteries in an abnormal state, insufficiently purified and imperfectly cleansed of the impurities which it took up on its return journey.

These impurities if returned to the system will certainly manifest in some form of disease, either in a form of blood disease or some disease resulting from impaired functioning of some insufficiently nourished organ or tissue.
The blood, when properly exposed to the air in the lungs, not only has its impurities consumed, and parts with its noxious carbonic acid gas, but it also takes up and absorbs a certain quantity of oxygen which it carries to all parts of the body, where it is needed in order that Nature may perform her processes properly.