A type of temporary tattoo is a "henna" tattoo. These are very popular among tourists in vacation destinations who want to return home with the memory of their trip inked on their skin. A natural henna tattoo is created from the orangeish dye produced by the plant's leaves. This dye binds with skin and takes weeks to wear off, which makes it a great candidate for temporary tattoos. The only problem with henna is its orange hue. Understandably, those looking for a temporary tattoo want it colored black. Enter "black henna".
First off, there is no such thing as henna dye that is black in color. Anything marketed as "black henna" isn't henna whatsoever, it's usually hair dye. The problem with using hair dye as a basis for temporary tattoos is one of its chemicals, para-phenylenediamine (PPD). This chemical can cause severe allergic reactions among people, including permanent scaring from blisters, lingering sensitivities to chemicals, and painful itching. When PPD is absorbed by the skin, it decreases the body's tolerance to effectively adjust to the toxin. One black henna tattoo might not cause any significant harm, but a second or third (or simply having your hair dyed down the road) could trigger an intense allergic reaction.
The PPD concentrates in hair dye are generally around 6%. Instructions warn applicants not to allow the dye to touch the scalp, and to rinse off the dye quickly. PPD levels in black henna pastes can reach upwards of 60% and are left on the skin for around thirty minutes. In most western countries, PPD is actually illegal for use on skin. While black henna tattoos still may be offered in tourist areas of western countries, they are much more popular in second world areas such as Costa Rica, Mexico, the Dominican Republic, or the Caribbean Islands.
Before deciding to get a temporary tattoo, fully investigate whether or not PPD is used in the dye for the paste. Remember, there is no such thing as black henna. If you want a henna tattoo, go for it; just note that the hue of the paste should be a reddish orange.