Anyone familiar with alchemy will recognize the theme of the black sun, also called sol niger or umbra solis. In distinction to the astronomical sun, the alchemists had a sun that shines black light.
So much did the alchemist sense the duality of his unconscious assumptions that, in the face of all astronomical evidence, he equipped the sun with a shadow: “The sun and its shadow bring the work to perfection.” Michael Maier, from whom this saying is taken, [. . . then cites] the classical saying of Hermes: “Son, extract from the ray its shadow,” thus giving us clearly to understand that the shadow is contained in the sun’s rays and hence could be extracted from them (whatever that might mean). Closely related to this saying is the alchemical idea of a black sun, often mentioned in the literature. This notion is supported by the self-evident fact that without light there is no shadow, so that, in a sense, the shadow too is emitted by the sun. For this physics requires a dark object interposed between the sun and the observer, a condition that does not apply to the alchemical Sol, since occasionally it appears as black itself. It contains both light and darkness. (Carl Jung, Mysterium Coniunctionis, CW 14, par 117)
As we can see, the alchemical Sol, as a personification of consciousness, is ambivalent. It shines bright light and dark sun rays at the same time. Jung intuits the physics of this phenomenon in the following way.
No psychic content can become conscious unless it possesses a certain energy-charge. If this falls, the content sinks below the threshold and becomes unconscious. The possible contents of consciousness are then sorted out, as the energy-charge separates those capable of becoming conscious from those that are not. This separation gives rise on the one hand to consciousness, whose symbol is the sun, and on the other hand to the shadow, corresponding to the umbra solis. (ibid., par 152)
For Jack to be able to sharply detect “the real inner demonic quality of the light“, it means that the symbol of the black sun is constellated in his unconscious and projected onto any source of bright light. In other words, the negatives of his photographs allow him to perceive and experience the black sun nature of his own consciousness.
Symbolically speaking, light is attributed to consciousness. Therefore, Jack’s murderous consciousness for art’s sake is a form of intense light, albeit a destructive one. This destructive quality of consciousness is thus naturally associated with the nigredo (blackness) and putrefactio (decay) attributed to the black sun.
The problem here is that the dark light is not a personalistic pattern but one of the extreme ends of the ambivalent nature of consciousness. In this case, the presence of the black sun shows that Jack’s ego is unconsciously identified with it. This means that his ego is powered-up by impersonal, archaic, untransformed archetypal energy, a state of inflation whose self-destruction is certain.