Kali is black to represent Her transcendental nature.
Just as all colors disappear into black, all names and forms disappear into Her.
Maha Nirvana Tantra
In order to truly know ourselves, Yogis strive to release attachment to all names and forms because these are temporary and pure consciousness is perpetual.
Kali's tongue urges us to savor ALL the flavors of life on planet Earth. Gain and loss, success and failure, these are different flavors and we miss out on the fullness of the human experience when we push some flavors away.