‘Black clouds’ are the nigredo (Mylius, p. 234, and ‘Tractatus aureus,’ Ars chemica, p. 15).
Jung establishes the black cloud as a canonical alchemical symbol for the nigredo, citing Mylius and the Tractatus aureus as authoritative sources within the broader symbolic constellation of cloud imagery in alchemy.
, Mysterium Coniunctionis: An Inquiry into the Separation and Synthesis of Psychic Opposites in Alchemy, 1955thesis