How can you tell black henna from narmal henna?
I suppose you mean in powder form, Henna is never black so you
won`t be able to buy it as a powder. People mix Henna with
chemicals to get "black henna" which is very harmful and should not
be put on skin. Sometimes people sell Indigo for hair and call it
black henna but that is nonsense, you cannot use Indigo all by
itself to achieve a black colour. You won`t find black henna powder
anywhere, so there`s no need to be worried about that.