Sometimes this is to protect the skin against the sun, sometimes for the wedding, and sometimes for simple embellishment. It depends on the culture and traditions of the area.
There are many ways for mehndi henna, but more common to use some kind of glue to keep the henna on the skin long enough to stain and acid to set it up.
Sometimes other ingredients containing tannins or other natural colorants, such as coffee, are added to the mixture to make it darker. A plain, natural henna stain is light brown orange.
Mehndi Designs 2014
The black tea will make it a bit rich brown. Freshly ground coffee will make it brown a bit rich. Beet juice will make it red. Indigo will make it a bluish hue, and the hulls of black walnut will make it dark brown.
To make a black stain takes indigo and black walnut hulls safely. If you have anything else in it, be very cautious. With any chemical, even if it is natural, be careful to do an allergy test before applying.