Nigredo

Nigredo, or blackness, in alchemy means putrefaction or decomposition. The alchemists believed that as a first step in the pathway to the philosopher's stone all alchemical ingredients had to be cleansed and cooked extensively to a uniform black matter.

In analytical psychology, the term became a metaphor 'for the dark night of the soul, when an individual confronts the shadow within'.

Read more about Nigredo:  Jung, Cultural References