Not everybody is aware whether Lord Krishna was the embodiment of Maha Kali or of God Vishnu. Lord Krishna is the 8th incarnation of Vishnu, but according to the Kalka mythology, he was the embodiment of not Vishnu but Kalka Mata, Maha Kali or Ma Kali and Radha, not the nature of Goddess Lakshmi, but the incarnation of Mahadev.
According to Devi Purana, Bhagwan Mahadev (Shiva) was born as a daughter Radha. At the same time, Krishna’s 8 Patranian Rukmini, Satyabhama, etc. were also part of Mahadev. According to Goddess Purana, Lord Vishnu incarnate as Balram and Arjuna. There is a temple in Vrindavan, where Krishna is worshipped as Maha Kali. They are called Black goddesses.