Isis was called the “Black One” because of her association with fate and the mysteries of death.
The ancient name for Egypt was “Kemi,” which translates “Black Earth.”
The Arabs called Egypt “Al-Kemi” and the ancient art of alchemy practiced in the ancient Near East and medieval Europe was probably derived from this name.
Carl Jung describes the blackness of Isis and in this passage, relates her identity to Sophia:
The cognomen of Isis was . . .the Black One. Apuleius stresses the blackness of her robe . . . and since ancient times she was reputed to possess the elixir of life as well as being adept in sundry magical arts.
She was also called the Old One, and she was rated a pupil of Hermes, or even his daughter. She appears as a teacher of alchemy . . . She signifies earth, according to Firmicus Maternus, and was equated with Sophia . . . She is . . . the vessel and the matter of good and evil.
She is the moon . . .”the One, who art All.” (Carl Jung, Collected Works Vol.14, 14-15)