You must not have read the whole story:
“Straight away he (the doctor) identified it as black henna. I didn’t realise it was different to any other type of henna. ‘He explained it contains chemicals which aren’t meant to be used and told me that it’d burnt her skin.”
The people who sell the henna that I use offer it in different colors for differing shades of hair (some people dye the hair on their head with it). I use one that results in dark brown. They also have henna for black hair. Maybe it is the "black henna"?