I really don't think it's possible. I have tried every single method I could think of, both natural and not. What I did end up doing is eventually turning my hair into a nasty, gummy, melted slop that had to be cut from APL to mid-neck and still didn't lighten any indigo. Good luck to you though...
Some of themethods I tried work well on henna alone and even on hennindigo, but the two-step henna + indigo, that you, and I wanted to lighten, ya, not happening for me. I did manage to turn my indigo quite green by using bleach the first time, it was using bleach multiple times that gummified it.
honey/coconut milk/olive oil
peroxide 3%, 6% and 9%
peroxide/bleach - this one finally killed the hair
*probably other combo's I forget to mention