Krishna kali deity is worshiped in Bengali tradition. Both of them have opposite nature, pastimes and way of worship. It seems impossible for both to unite. Both Shakti and Vaishnav traditions have a hard time accepting each other because of their lifestyle which is completely opposite. In the 14th century Chaitanya Mahaprabhu, founder of Gaudiya Vaishnavism propagated worship of Lord Krishna. In the 18th century devotees like Rama Krishna Paramhamsa propagated worship of Kali. Therefore, Bengali tradition is influenced by both Krishna and Kali worship.
There is an interesting popular story regarding Krishna Kali. Every night RadhaKrishna met in the forest of Vrindavan. Radharani would leave her house and slip out. Once her sister in law, Kutila saw Radha Krishna together. She ran to her brother and mother to tell the truth. Radharani’s husband and mother in law was infuriated. They ran to the place mentioned by Kutila. They came too close to the place where Radha Krishna was and both of them saw them.
Suddenly Krishna turned into Kali and Radharani started worshiping Kali. When Radharani’s husband and mother-in-law came they saw Radharani worship Kali. Both of them scolded Kutila. Kutila promised that she saw Radharani with Krishna. Radharani’s husband is a great devotee of Kali therefore glorified Radharani and was enraged with sister Kutila.
One incident took place near Jagannath Puri, where a rich man lived. His name was Karta. He wanted to take Vaishnava initiation from a pandit. After a few months everyone criticized that the pandit was a devotee of Kali and not a Vaishnava. Few people came to Karta and took him to the place where he worshiped Kali. Later at night they took Karta to the place. The pandit saw him near to him. He was in shock and he prayed to Kali to help him in need. When Karta came near, the deity turned into Krishna. It is a beautiful pastime of Krishna and Kali showing them to be one and no different.