Talk about Goddess Kali and the image that comes to our mind is that of a fierce woman with blood dripping tongue. But why does Goddess Kali drink blood? Demons or Asuras through severe austerities obtain boons from Brahma. The demons often try to attain boons of immortality but Brahma often denies it. Instead the demons are given death under specific circumstances. The Asura Raktabeeja obtained a boon from Brahma that every drop of blood that falls on earth from his body will produce more demons.
The Devas could no longer fight Rakthabeeja and were on the receiving end. Finally, Mother Durga appeared as the terrifying Goddess Kali, who sucked up every blood that fell from Raktabeeja.
After destroying Raktabeeja, Goddess Kali remained blood thirsty and a horror to humans. Finally, Lord Shiva had to come down to earth to control her.
This is the reason why images of Goddess Kali are shown with blood red protruding tongue.