The black sun as pictured in the Putrefaction emblem of Philosophia Reformata (Johann Daniel Mylius)
Sol niger (black sun) can refer to the first stage of the alchemical magnum opus, the nigredo (blackening).
In a text ascribed to Marsilio Ficino three suns are described: black, white, and red, corresponding to the three most used alchemical color stages. Of the sol niger he writes:
"The body must be dissolved in the subtlest middle air: The
body is also dissolved by its own heat and humidity; where the
soul, the middle nature holds the principality in the colour of
blackness all in the glass: which blackness of Nature the ancient
Philosophers called the crows head, or the black sun.
— Marsilius Ficinus, "Liber de Arte Chemica"
The black sun is used to illuminate the dissolution of the body, a blackening of matter, or putrefaction in Splendor Solis, and Johann Daniel Mylius’s Philosophia Reformata.
