First indigo, then henna -two steps method, has anyone ever tried?

I've only heard that you shouldn't do it this way, but no one has told what happens if you do. I'm wondering if there's any way to get more blue out of indigo. Because usually it only gives the black/dark tint if you use it to hennaed hair.

I've used straight indigo on ash blonde hair, it was a unattractive blueish grey. I didn't try using henna over it.

Indigo can have a hard time, sticking to the hair on it's own. Henna first, helps indigo with this.

Thanks for the answers! I now tried this myself and it worked just fine. I had there Henna Indigo dye under already (I must keep dyeing my gray roots all the time) but now my one step henna-indigo-mixture made the color very dark and too colourless and greyish brown somehow. So I dyed it with pure henna afterwords, and it gave my hair nice red tint.