Every year, I see countless news reports about this and still there are business proprietors pushing this product, even going so far as telling their clients that it's better than regular henna and perfectly safe. This is a lie - natural henna is a powder derived from a plant and, in its purest form, causes virtually no allergic reactions. Black henna, however, is natural henna that has been altered by adding a dangerous, toxic chemical known as PPD - it's often found in black hair dyes and is typically safe to use on the hair but is widely known to cause severe allergic reactions when it comes in contact with skin. Black henna or any product containing PPD should not be used on the skin, ever.
For more information on henna, black henna, and how to enjoy henna art safely, check out the following articles: