Nigredo, the blackening, is the first of the colour-stages through which the alchemical Great Work (magnum opus) was said to pass. It names the first stage, in which the matter is broken down, putrefied and reduced to a black, formless mass.
The colour-stages of the Work
Symbolically Nigredo marked the death and dissolution of the old form, and, in the inner reading, the confronting of one's own darkness. The classic sequence, nigredo, albedo, citrinitas, rubedo, describes the transformation of base matter into the perfected end of the Work through a succession of colours seen in the vessel.
The earliest named source for a colour-stage sequence in matter is the Greco-Egyptian alchemist Zosimos of Panopolis, writing around the turn of the fourth century CE, whose visionary treatises describe a figure dismembered, blackened and boiled as an image of the material transformation he was pursuing; Latin alchemical writers of the fourteenth to seventeenth centuries, working in a tradition descended from that Greco-Egyptian material by way of Arabic translation, then elaborated the sequence into the fuller four-stage form generally cited today. The stages carried at once a chemical and a spiritual meaning, and were read by the depth psychologist C. G. Jung, in his 1944 study Psychologie und Alchemie, as an allegory of inner transformation, the nigredo in particular corresponding to what he called the encounter with the shadow. The stage is described here within that documented tradition of European alchemy and its later interpretation.
Sources The Secrets of AlchemyLawrence M. Principe with Psychology and AlchemyC.G. Jung