Pakistani mehndi stains the skin of an orange brown color, not black colored stains(unless you add to the mix some chemical that color).

It is extremely unusual for someone to have some kind of adverse reaction to natural Pakistani mehndi paste (only natural products), since it is one of the safest cosmetics that have been used through history.

If you develop any type of allergic reaction when using it, you should generally not the Pakistani mehndi itself, but to some pigment color added to make it black or something that was mixed natural Pakistani mehndi powder to create a paste.