"Black henna", sometimes referred to as "kali mehndi" or "piko" is henna that that has been adulterated with the transdermal toxin, p-paraphenylenediamine (PPD for short) mainly used as a hair dye. In some cases this so-called henna doesn't even contain any henna at all. PPD is associated with bladder and blood cancers, liver failure, and fetal harm. It is currently illegal for use on skin in the USA and some other countries.
Colored Henna and commercial cones are generally unsafe. Reputable henna artists make their own henna paste, and do not buy commercially manufactured cones from India or Pakistan. If a cone is shelf-stable (doesn't require refrigeration) that is one clue that it contains artificial dyes or preservatives. Some chemically adulterated brands that are currently on the FDA import-alert are Kaveri, Neha, Prem, Supreme, Hemani, Rani.
Getting a Dark Stain with Natural Ingredients is possible. Probably the most popular way to get a nearly black stain naturally is to use Jagua-henna. Jagua is a tropical fruit with a natural blue dye and when mixed with henna, the stains are extremely dark and long-lasting. Check out this video tutorial for a fast and simple recipe.
How to make your own natural Jagua-henna for a nearly black stain:
All of the ingredients you need are available at Henna Guru.
Do you think you are safe because you are getting a henna design from an "ethnic" or "traditional" artist? Think again. Many places in India (as well as the rest of South Asia), the Persian Gulf, and the African continent are now using "black henna" and claiming it is traditional and natural. It's not natural, it's not traditional, and it's not safe. When I was traveling in India, I met a cute 13 year old girl who was doing black henna at the beach (in fact she had painted a black mustache on her 3 year old little brother). She wasn't fluent in English, but she knew the word "allergy". In fact she had no knowledge whatsoever of how natural henna worked. When I did a design for her in natural henna, she washed the paste off after 20 minutes and wondered why the stain was orange. Don't assume that "traditional" artists are educated just because henna is "part of their culture". No one is born knowing how to do henna - the knowledge must be acquired. In many parts of Africa, "black henna" is now the norm, but this doesn't make it safe. Additionally, "black henna" is also being offered in tourist destinations such as Mexico, Costa Rica, Brazil, and other beach or "Spring Break" locations, and even in the USA!
Always ask your artist what the ingredients in their henna paste consist of. Ingredients should all be recognizable - henna/mehndi leaf powder, water or lemon juice (or possibly jagua juice), essential oils (usually tea tree or lavender), and sugar. Terms to watch out for are "mehndi oil", "black clove oil", sodium picramate, metallic salts, PPD, or "henna stone". Vague terms like "mehndi oil" often signify the addition of toxic solvents like benzene (gasoline) or kerosene. If the mixture smells like ammonia, gas, or chemicals, this is another sign the artist could be using unsafe ingredients.
Real henna needs to be left on the skin for several hours or more in order to give a great color, and the stain starts out orange and oxidizes to a reddish brown tone over 48 hours. Occasionally, on the palms and soles of the feet, henna can reach a nearly black color, but shining a light on it, you will see it is actually a very rich brown or red. Sometimes, repeated applications of the henna can also achieve a similar effect, but once again, the the stain is not instant and requires many hours to achieve. The hands and feet will always stain darkest, while the torso or other body parts will be fainter, as shown below.
Ok, so you think just this once isn't going to hurt you? What if your beautiful mehndi turned out looking like the image below? What if you then had a lifelong allergy to black dyes, including hair dye, printer ink, or even black socks or underwear?
"I've been using it for years and I'm fine". Well, did you know that PPD is associated with bladder cancer, renal failure, and fetal harm? Symptoms may take years to develop, but can be fatal. Do you really want to take chances like this when natural henna can give you a beautiful, rich color?
You could die from PPD exposure:
Yup, you read that right. This woman first got a black henna in Dubai some years ago. Over time, her allergy to PPD intensified and she eventually died from anaphylaxis, as explained in this BBC article.
Here are some resources that I got my information from (in addition to extensive anecdotal evidence I have acquired from speaking to thousands of henna recipients over the last decade):