Black Henna Warning
What is 'Black Henna'?
There is no such thing as black henna.
Although there are safe ways to get temporary black body art, most "Black henna" may contain p-phenylenediamine (PPD), which can stain skin black quickly, but can cause severe allergic reactions and permanent scarring.
PPD is illegal for use on skin in western countries, and is specifically forbidden by the FDA, and may only be used in hair dye when in concentrations of less than 6% and instructions not to let it touch the skin. The most frequent health consequence following "Black Henna" tattoos, where PPD is almost twice the allowed concentration and left in direct contact with the skin, is sensitization to hair dye and related chemicals, causing and life-long consequences including life-threatening reactions to:
Use of PPD is widespread, particularly in tourist areas. Reactions to PPD and "Black Henna" normally occur anywhere between 3 - 12 days after application, meaning that many tourists are already headed home before they are aware that anything is wrong.
There are two major indicators of PPD Black Henna:
For more details about "Black Henna", PPD and the related health risks, read this Wikipedia article and it's related references and sources.
Black Henna Alternatives
That's not to say that black temporary tattoos don't exist in the market!
Other options include Indigo, Jagua, Harquus, and Glitter tattoos.
Alternatively, there are natural ways to get the most out of your henna without exposing yourself to chemicals:
Darker for Longer
The longer the henna is in contact with your skin, the further down the dye will travel. This will make the stain darker and also make it last longer.
Wrapping your design in something that will trap heat (such as kitchen towel or cling film) will keep the skin warm, speeding up the rate the dye enters the skin. Make sure it's dry though, or you could find yourself with some dark smudges, too!