Black henna tattoos last longer than authentic red henna tattoos because they contain black hair dye made from coal that contains some noxious chemicals.
Why you should avoid ‘black henna' tattoos
You may be familiar with henna, a reddish-brown coloring made from a flowering plant that grows in tropical and subtropical regions of Africa and Asia. Since the Bronze Age, people have used dried henna, ground into a paste, to dye skin, hair, fingernails, leather, silk and wool. This decoration—sometimes also known as mehndi—is still used today around the world to decorate the skin in cultural festivals and celebrations.
However, today so-called "black henna" is often used in place of traditional henna. Inks marketed as black henna may be a mix of henna with other ingredients, or may really be hair dye alone. The reason for adding other ingredients is to create a tattoo that is darker and longer lasting, but use of black henna is potentially harmful.
That's because the extra ingredient used to blacken henna is often a coal-tar hair dye containing p-phenylenediamine (PPD), an ingredient that can cause dangerous skin reactions in some people. Sometimes, the artist may use a PPD-containing hair dye alone. Either way, there's no telling who will be affected. By law, PPD is not permitted in cosmetics intended to be applied to the skin.