henna is not black

Henna does not come in different colours.  It's a plant with 1 dye molecule that stains the skin 1 colour. The range of colour from orange-reddish brown to dark cherry depends on skin thickness, oxidation, temperature and body chemistry.

Fake ‘Black Henna’ is concentrated para-phenylendiamine (PPD) based hair dye. This is illegal to use on the skin in Canada because of its severe toxicity:
it is known to cause tumors, organ failure, permanent scaring, muscle deterioration, etc... PPD has a delayed symptomatic reaction of 3–10 days; feeling ok while the 'tattoo' is on does not mean it's safe.

Within seconds the toxins from PPD enter the blood stream. It is a strong sensitizer, meaning every time a person comes into contact to PPD, the body will react more violently to it. After having concentrated PPD applied to the skin, people often react to synthetic hair dyes, black clothing & cosmetics.

PPD damage isn't always obvious (scaring, eczema, asthma), the majority of the damage is internal (cancer, swelling, organ failure, mutated cells, muscle necrosis causing death, eye & face irritation, etc...). A classic example is a person feeling exhausted after coming back from a tropical vacation: they think it's from travel, when really the 'black henna' applied on the second day of the trip is damaging the body. To worsen the situation, the metabolic residue from the liver trying to clean out the PPD is even more damaging than the whole molecule.

How to tell is it's PPD? Before letting anyone henna you, ask them if they made their henna paste & what is in it. If they bought their paste from a store or brought it over from another country, don't trust it. Then ask what colour it will stain the skin. If they say black, run away. If they say dark brown, well, it still could be real henna, but they aren't taking the time to properly explain the variations in henna stain colour... so the final question can be how long does it take for the henna to stain a dark colour? If they say to just leave the henna paste on for 15 - 20 minutes, it's definitely a chemical paste. Henna paste needs to be on the skin for 4 - 8 hours (depending on the temperature) for a dark stain.

Reporting PPD:
It is a crime to apply PPD "black henna" to the skin. Unfortunately, the government relies on the public to report offenders. If you see someone applying PPD body art, or selling "black henna" cones, report them to Health Canada: 1-866-662-0666, Tor.Prodsafe@hc-sc.gc.ca