The unity of Kali and Krishna can be discovered through analyzing
their bija mantras and their names:
Kali = krIM
Krishna = klIM
First of all, they only differ by one letter.
Second, if you take each bija letter by letter, you can arrive at the
correspocding name of the other:
klIM (Krishna bija) = ka + la + I + M ~ KALI
krIM (Kali bija) = ka + ra + I + M ~ KRISHNA (First 3 letters anyway.)
They are not exact, but neither are these two deities the same in
Their INNER unity is shown in their beginning, "ka"; their middle,
"I"; and their end "M." The one letter difference changes the entire
outward form and function of the deity ("ra" is fire = transformation
= Kali and "la" is Earth or Krishna coming to Earth).
Further, "krishna" means dark, and Kali is darkness Herself.
Finally, Kali has been translated as "She who takes away the darkness"
and Krishna was Vishnu's avatar who descended in the Dvapara Yuga to
take away the darkness of that age. Surely Kali was the shakti behind
AUM kAlyai namaH