long reader, not an avid poster, After doing a lot of research on the subject, (a lot of research) I found out that yes in some cases it is true. Henna labeled "Black Henna" in better Indian Stores, like Kalustyan's (NYC, Lexington/28th Street) is actually indigo. Apparently back in the day and until today in certain areas of the community Indigo is marketed as "black henna" which is why so many of us get the 
look when we ask for Indigo in the Indian Markets. It's known as "Black Henna"
BUT LOOK OUT:
"If “Black Henna†powder is a green plant powder and smells like frozen peas, its probably indigo. Mix some of the powder with water. If the surface of that mix turns blue in 20 minutes, it’s indigo! Indigo will not hurt you if you put it on your skin. It won't stain your skin black."
There are other Dyes that are marketed as Black Henna that are dark in color brown or black. These types are very dangerous and are known to scar skin.
Just a Heads Up.
look when we ask for Indigo in the Indian Markets. It's known as "Black Henna" BUT LOOK OUT:
"If “Black Henna†powder is a green plant powder and smells like frozen peas, its probably indigo. Mix some of the powder with water. If the surface of that mix turns blue in 20 minutes, it’s indigo! Indigo will not hurt you if you put it on your skin. It won't stain your skin black."
There are other Dyes that are marketed as Black Henna that are dark in color brown or black. These types are very dangerous and are known to scar skin.
Just a Heads Up.
