As far as I'm concerned, Indigo is the only natural hair dye that I'm aware of. Bigen has PPD in it; therefore, it is not chemical free. They just promote no ammonia or peroxide in the product. Please read the following link, it provides additional info on ppd and there are legitimate sources that are referenced that you can follow up with just incase you want to do additional research.
http://www.hennaforhair.com/ppd/index.html
I was going to do bigen because I was looking for a chemical free alternative for dying my hair but after I read about PPD, and learned that Bigen has it, I began to change my mind. If you(or anyone) chooses to use Bigen, please do a strand test because not everyone gets a negative reaction to it. But the stories I've read with respect to people's negative reactions, I was scared off from using. That sealed the deal for me.
Guesswho, I used indigo and my hair came out super jet black! I love it! I put 2 teaspoons of salt in my mix to intensify the black. I've had no issues of staining whatsoever. I used it about 3 weeks ago and it's still jet black. I've been using Sunsilk Black Shine shampoo to maintain the color. The only downside is that it is messy and it is a lengthy process when you combine the time need to do the henna treatment in addition to the indigo. Anyhow, good luck on your search.