In alchemy, nigredo, or blackness, means putrefaction or decomposition. Many 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.
For Carl Jung, 'the rediscovery of the principles of alchemy came to be an important part of my work as a pioneer of psychology'. As a student of alchemy, he (and his followers) 'compared the "black work" of the alchemists (the nigredo) with the often highly critical involvement experienced by the ego, until it accepts the new equilibrium brought about by the creation of the self'. Jungians interpreted nigredo in two main psychological senses.
The first sense represented a subject's initial state of undifferentiated unawareness, 'the first nigredo, that of the unio naturalis, is an objective state, visible from the outside only...an unconscious state of non-differentiation between self and object, consciousness and the unconscious'. Here the subject is unaware of the unconscious; i.e. the connection with the instincts'.
In the second sense, 'the nigredo of the process of individuation on the other hand is a subjectively experienced process brought about by the subject's painful, growing awareness of his shadow aspects'. It could be described as a moment of maximum despair, that is a prerequisite to personal development. As individuation unfolds, so 'confrontation with the shadow produces at first a dead balance, a standstill that hampers moral decisions and makes convictions ineffective or even impossible...nigredo, tenebrositas, chaos, melancholia'. Here is 'the darkest time, the time of despair, disillusionment, envious attacks; the time when Eros and Superego are at daggers drawn, and there seems no way forward...nigredo, the blackening'.
Further steps of the alchemical opus include such images as albedo (whiteness), citrinitas (yellowness), and rubedo (redness). Jung also found psychological equivalents for many other alchemical concepts, with 'the characterization of analytic work as an opus; the reference to the analytic relationship as a vas, vessel or container; the goal of the analytic process as the coniunctio, or union of conflicting opposites'.
- In the alchemical literary discourse Hydriotaphia, Urn Burial(1658) the meditative nigredo stage is described as, lost in the uncomfortable night of nothing by the physician-philosopher Thomas Browne 
- Shakespeare's sonnets are dense with the symbolism of the "nigredo"..."ghastly night"'.
- W. B. Yeats in his alchemical stories 'introduces the alchemical phase of the nigredo. The narrator begins "to struggle again with the shadow, as with some older night"'.
- The chamber opera Nigredo Hotel by Nic Gotham and Ann-Marie MacDonald is titled after the Jungian concept of nigredo. The opera's themes are heavily influenced by Jung's work.
- In the popular Japanese light novel series Overlord, one of the ¨Area Guardians¨ is named Nigredo. She has two younger sisters named Albedo and Rubedo.
- Chemical History Tour, Picturing Chemistry from Alchemy to Modern Molecular Science Adele Droblas Greenberg Wiley-Interscience 2000 ISBN 0-471-35408-2
- Roberte H. Hopeke, A Guided Tour of the Collected Works of C. G. Jung (Boston 1989) p. 165
- C. G. Jung, Man and his Symbols (London 1978) p. 40
- Hans Dieckmann, "Shadow (Analytical Psychology)"
- Paul W Ashton, From the Brink 9London 2007) p. 231
- Gerhard Adler, Studies in Analytical Psychology (London 1999) p. 19
- Ashton, Brink p. 231
- Jung, C. G. Psychology and Alchemy 2nd. ed. (Transl. by R. F. C. Hull
- C. G. Jung, Mysterium Coniunctionis (London 1963) p. 497
- Christopher Perry, in P. Young-Eisendrath/T. Dawson eds., The Cambridge Companion to Jung (Cambridge 1977) p. 152-3
- C. G. Jung, "Psychology of the Transference", Collected Works Vol 16 (London 19540 p. 279
- Hopeke, A Guided Tour" p. 164-5
- M. C. Schoenfeldt, A Companion to Shakespeare's Sonnets (2007) p. 414
- William T. Gorski, Yeats and Alchemy (1996) p. 85