There are several things marketed as "Black Henna", and some things believed to be "Black Henna".  Some are very dangerous.  Some are harmless.

When para-phenylenediamine is used to make black temporary tattoos, often called “black henna”, it can cause blistering, open sores, scarring, and lifelong health problems. 
 

1) Some people make a black temporary tattoo they call "Black Henna" with synthetic black hair dye containing para-phenylendiamine. 

Para-phenylendiamine based black hair dye should never be put straight on your skin, plain, or mixed with other material.   Synthetic Black Hair Dye is illegal to put on skin, because that is not an approved use.  Even when this dye is applied to hair, people must wear gloves, and they try to not get it on the scalp!  PPD, Para-phenylendiamine can seriously injure people.  Para-phenylenediamine is a strong sensitizer, transdermal toxin and potential carcinogen.  Many people get itching, blistering and hair loss from using black hair dye on their hair. 

See http://www.hennapage.com/ppd2/ppdmed.html      for a list of medical journal articles on just how dangerous PPD can be!
See http://www.hennapage.com/ppd/2ppdburngallery.html  to see what these injuries look like.
 

 2) "Black Henna" was once a term for indigo, when it was sold as hair dye. 

In the 1800's there was no synthetic hair dye.  Henna and indigo were used to dye hair.  Henna leaves and twigs that had no, or very low, concentrations of Lawsone (the tannin produced by the henna plant) were sold as neutral henna.  Henna leaf buds that had high concentrations of Lawsone were sold as Red Henna or Henna.  Indigo was marketed as "black henna". If you dye your hair with henna first, and then dye over that with indigo, your hair will be dyed black.  A plant known as "wasma" was also mixed with henna to dye hair black.   If you see a package of "black henna" in a Middle Eastern or Indian grocery it is probably indigo. If you see a package of black hair dye from an American cosmetic company, it has some form of para-phenylenediamine in it. 
 

 3) National Geographic has shown many pictures of people in Amazonia and Africa adorned with black body art. 

People see this body art and think it might be "black henna".  It is not henna. This body art is created with carbon and the sap of two species of unripe figs that are rare and grow only in the rainforest. 
 

4) There are safe and effective traditional techniques of making natural henna go dark red or dark brown, even nearly black. 

Heat and the addition of essential oils such as Tea Tree oil are the safest and most effective darkeners of henna. These may make nearly black stains on hands, but on the rest of the body, the stains will be brown.  Henna never leaves “black” stains on arms, legs, backs and bellies! 

 The safe techniques are listed on other areas of this site, Mehandi.com: http://www.mehandi.com and The Reverend Bunny's Secret Henna Diaries: http://reverndbunny.sphosting.com/reverndbunny_sphosting_com.html    

Do you want to find out what is in a box of Henna or “Black henna”?

Look in the box.  If the powder is green, and appears to be ground-up plant leaves, it's probably henna or indigo.  If the powder is coffee colored or black, it's probably para-phenylenediamine based hair dye.

Take a little of the powder, and mix it with water.

If  “Black Henna” powder is a green plant powder and smells like frozen peas, its probably indigo.  Mix some of the powder with water.  If the surface of that mix turns blue in 20 minutes, it’s indigo!  Indigo will not hurt you if you put it on your skin. It won't stain your skin black.

“Black Henna” mixes with PPD are usually dark brown or black.  Sometimes there is some henna in the mix, sometimes not. Some of these mixes don’t have much odor, others have an awful chemical smell.  Mix some of that powder with water.  In 10 minutes, black dye will drain out of the mix. 

Henna is a green plant powder that smells like hay.  Mix that with water and wait a few hours.  The surface will turn brownish.

Henna is a green plant powder.  If its  mixed with water will turn brown in a few hours.

PPD "Black Henna" is a blackish brown or black powder.  If its mixed with water, black dye will drain out.

"Black Henna" indigo is a green plant powder.  if it's mixed with water, it will turn blue in half an hour.  Want to see what this looks like so you can tell the difference between PPD, henna and indigo?  See: http://www.mehandi.com/PPD/indjamppd.jpg