Character of the tongue: The whole tongue is bluish and purplish evenly or the blue-purple color is present with the normal tongue color.
Clinical significance: It indicates unsmooth movement of qi and blood, and blood stasis
Main causes: 
a. Interior yin cold and deficiency yang result in qi obstruction and blood stasis.
b. The pathogenic heat enters the nutrient and blood aspects and consumes nutrient to induce unsmooth movement of qi and blood. 
c. Failure of lungs to diffuse and depurate or liver to free coursing leads to qi obstruction; qi deficiency fails to activate or move blood.
d. External forces rupture the blood vessels and the blood runs away to stay above the tongue.
e. The congenital heart disease or food poisoning.
Also learn Relationships Between Spleen and Body-Sensory Organs and Orifices.