A number of people when traveling around Indonesia and particularly Bali, enjoy the exotic creations of temporary tattoo artists. They are on every main street in the holiday hotspots and are also found at school fairs and other festivities in and around the cities.
The main ingredient used for these temporary tattoos and/or Mehndi (the traditional art of henna painting in India and the Middle East) is henna. Pure, natural henna gives a chocolate brown or deep mahogany red stain on the hands and feet. On other parts of the body the stain can be tea-coloured; the shades can vary depending on the individual and other factors such as skin colour. Henna's dye component, hennotannic acid, does not pass through into the dermis (living cells) layer of the skin, it only stains the dead cells in the epidermis. Hennotannic acid naturally makes the brick/red/brown stains and it usually takes many hours to get a good henna stain on the skin, and is relatively safe. Allergic and irritant reactions are very rare. To make the dye, henna leaves are dried and finely ground and the powder is filtered; the artist then mixes the fine powder with an oil (such as eucalyptus oil) and other liquids (lemon, water, or tea), making a thick paste. This may sometimes appear to be black when wet and freshly applied to the skin, but on closer inspection is unlikely to be so.
Henna, however, is not black and most "black henna" as it is called is natural henna adulterated with a toxic chemical dye to make it black and to speed up the dyeing process. There are more and more people who are having problems of skin reactions with so called "black henna" within Indonesia. The fact of the matter is that it is not the henna that is the problem but the toxic dye called para-phenylenediamine or PPD for short, that people are adding to the henna to produce the black stain on the skin. PPD may also be used on its own to stain skin black and it stains the skin absolutely black within 2 hours and the stain can last easily 2 weeks. PPD penetrates deep into the skin, reaching the dermis and passing into the blood stream and can be dangerous to your system and not only affect the skin where it has been applied.
While there are a few safe products out there, most "black henna" contains PPD, and is found in many black hair dyes; there are some black (and other coloured) henna stains that claim not to contain PPD but rather use other non-toxic colourants; individuals would need to check on the actual ingredients to determine this and the safety thereof.
The skin reactions to PPD "black henna" frequently resemble the photograph that has kindly been provided by a friend. P-Phenylenediamine can also cause delayed type hypersensitivity reactions on the skin exactly where the PPD "black henna" temporary tattoo was applied. Sometimes the reaction is mild, with a raised, itchy area where the "black henna" pattern was. More severe reactions include pustules, blisters, oozing sores, intense itching, and these often leave long-term scars on people. The onset of the reaction generally begins 1 day to 3 weeks after the PPD "black henna" has been applied. Sometimes the blisters come up while the black pattern is still visible, and other times the blisters appear when the pattern has faded. If someone has a hypersensitivity reaction one time to PPD "black henna", it will be more severe with each subsequent exposure.
It is of no use to "patch test" PPD "black henna" to see if you're going to have oozing 
sores and blisters, because the reaction can occur up to 6 weeks after exposure. Also, skin hypersensitivity reactions will be increasingly likely with each exposure to PPD. A person may have one PPD "black henna" tattoo with no reaction, but may have a severe reaction with the next PPD "black henna" tattoo. Even if there is no skin reaction to PPD, it is still a transdermal toxin and can be harming your internal organs without showing lesions on your skin!
Although rare, severe cases of immediate type hypersensitivity to PPD have been described in which the patients developed severe edema (swelling), irritation of the eyes and face and also difficulty in breathing.
PPD scarring may be permanent and post-inflammatory hypopigmentation may remain at the tattoo site (this means after the scarring and blistering has subsided, a long-lasting white ghost image of the tattoo remains).
If we cannot control the use of these toxic chemicals, there are ways to protect yourself from black henna and find responsible, traditional henna artists.
Unfortunately, it is usually our children who are the ones that wish to have the tattoos, but are also the ones who are particularly at risk.
How to recognise PPD-based "black henna":
- If the paste is greenish, and smells like spinach or a hay bale, it is probably natural henna. Sometimes reputable henna artists add natural oils or herbs that can spice up the fragrance and darken the color.
- If the paste is deep black or grayish, it is probably NOT natural henna and should be avoided. PPD paste is jet black.
- Ask how long it takes to stain and how long it lasts. If the answers are "just an hour or two" and "a week or more", it's PPD. If the henna artist tells you that you only need to leave the paste on for an hour or two, their henna probably contains PPD and should be avoided! Real traditional henna must stay on your skin longer to make a stain - usually 6-12 hours. The resulting color will be a reddish brown, not black.
- Ask how long it lasts. If the answer is "a week or more" then it most likely contains PPD.
- Ask what colour it will give. If the answer is "pure black", it's PPD. Real henna does NOT stain black. PPD does stain black! If the artist is offering "colored henna" ask to see a list of ingredients, and decide for yourself if the product is safe for you.
- Ask to see an ingredients list. If the artist can't supply one, or you don't like what you see, walk away.
Source : http://www.expat.or.id/medical/blackhennareactions.html
