Sung in the northern Indian state of Uttar Pradesh, Kajri has a popular legend associated with it. According to folklore of Mirzapur, a place in UP, a woman named Kajli had to live in separation from her husband, who lived in a faraway land. She would miss him all the time, but when thick clouds splashed monsoon showers across the land, the estrangement became unbearable for her. She is believed to have taken her petition to a certain goddess Kajmal with her wailing. The other origin story comes from the Hindi word kajal, meaning kohl. The colour black is related to the dark clouds of monsoon, which in this case, bring relief.
If folk beats and earthy melody interest you, listen to a selection of Kajri here.