First of all, there is no such thing.
When people advertise using "black henna," it is usually made from a synthetic chemical called para-phenylenediamine or PPD, which is commonly found in hair dye. This chemical is very toxic to the skin and can cause blisters and chemical burns. The result can be a permanent scar in the shape of your tattoo. An adverse reaction to PPD can also cause chemical sensitivity and organ damage, There is a reason why hair dye manufacturers recommend that you avoid getting the dye on your scalp and hands! Also beware of any substance labeled "mehndi oil". These mystery "oils" often contain kerosene or other solvents. Sadly, manufacturers will also add chemicals to pre-made branded brown henna paste to make it darker or prolong the stain. Professional artists make their own paste. Do not get henna from someone using a branded pre-made paste.
Henna only comes in one color and that is a shade of brown to reddish brown. If someone is using "black henna" and tells you to only leave the paste on for an hour or less, beware! Real henna stains take a good 4-6 hours or more to develop. When you remove real henna paste, the design will initially look bright orange for the first half day, then slowly darken to brown during the next 48 hours.
If you see someone offering "black henna," it is best to avoid it unless you know for certain that it is a safe substance. I have heard of many instances where the "black henna" artist will claim their product is safe when it's not. After all, they WANT YOUR BUSINESS. Some don't even know the ingredients in their own product or they 'got it off the internet". Don't be afraid to ask what is in the paste. If the artist cannot or will not give you an answer, stay away! It's not worth the risk.
For more info, visit the FDA's page on "black henna".
There ARE safe versions of substances the stain the skin a bluish-greyish color, but they are more than likely Indigo or Jagua and not henna. Indigo is a safe, naturally occurring dye that will stain the skin blue, but it will only last for a day or two at most. Juaga is made from a fruit and it also a safe, naturally occurring dye.
HennaMe paste is home made and contains all natural ingredients including henna leaves, sugar, lemon juice, cajeput oil and molasses. There are absolutely no chemicals in our paste! Here's what natural henna looks like: