Marvel's Iron Fist: 44 Easter Eggs & Reference You Must See

13. The Black Sky

When Colleen decides - with limited reason - to change her entire life and belief system and chooses Danny over her sensei and the entire fabric of her existence, Bakuto goes wild on her. He gets her students to exact revenge by taking her to the basement to be drained in a cage in the same weird procedure introduced in Daredevil.

The suggestion here is that she would be drained of her blood the same way those child victims were, in order to feed her essence to Elektra/Black Sky. Presumably, she now requires regular top ups.