Black henna is not natural henna. Instead, it's a toxic chemical substance called PPD that's used in manufacturing and in some hair dyes. Despite the fact that exposure to PPD can cause respiratory problems and that direct contact with it can cause severe skin irritation, PPD is sometimes used to make "black henna" tattoos that are blacker in color than the orangish or reddish stains produced by natural henna. PPD is not approved for use as a skin dye and it should be avoided.