i just bought a pack of (as it says on the pack) "Black Henna" from a foreign shop for $3.50 a 500g. I only felt safe buying it because it smelled of "green" and the how-to card said that it need be mixed with red henna to work (the ratio they clame is one part black henna, two parts red). i googled and found out it might be PPC at which point i tried to find out if it was.
It smells a bit like dried grass or gay (i have no idea what frozen peas smell like) and is a fine green powder - sounds like indigo so far, right? i can't find any trace of chemical smell in it.
But! It doesn't stain the skin at all. And it doesn't turn blue when mixed with water. It stays a greep, almost murky, forest green.