Natural Henna is prepared with an acid like lemon juice. In order to turn henna designs black is by treating them with ammonia. Ammonia was most easily found in the store or picking up a bottle of household strength ammonia.
“Please note, ammonia is potentially hazardous to ones skin, so read the following before you try to use ammonia on yourself or others! Personally I don’t use ammonia on my client. Most of the client is unaware of the side effect.”
Before you apply black henna, read the warning label on the back of the ammonia bottle! This only works well on palms and should only be tried by an adult with healthy unbroken skin. If the ammonia starts to burn or hurt, quit this immediately, and flush the skin with water and vinegar. Do not do this if you have sensitive skin, if you have broken skin, or if you are a child. Do a patch test on your skin with the ammonia before you do a large area! Use household ammonia, not laboratory ammonia. Do not use lemon ammonia, try to use the plainest household ammonia you can find. Household ammonia is 90% water. Don't get ammonia in your eyes, mouth, or on thin delicate skin. Ammonia is caustic, and can give you something like diaper rash or dishpan hands. It does not usually bother healthy adult palms and soles. It works much better on hands that have firm to hard skin than soft skinned hands. On very soft hands, this does not work properly, the skin must be firm and porous for this to work. Do this only in a well ventilated area.
“Do not apply on skin other than palms and soles! Ammonia/ Black henna on smooth skin hurts and will exfoliate your henna before it ever darkens! I can only advice you don’t use black henna, when natural henna is very dark and doesn’t have side effect and stay longer. There is no need to use Ammonia/ Black henna.”
Caution: Health Canada alerts Canadians not to use "black henna"
OTTAWA - Health Canada is advising Canadians that the use of the ingredient para-phenylenediamine (PPD) in "black henna" temporary tattoo ink and paste is unsafe. Allergic reactions to PPD include red skin rashes, contact dermatitis, itching, blisters, open sores, scarring and other potentially harmful effects. Allergic reactions to PPD may also lead to sensitivity to other products such as hair dye, sun block and some types of black clothing.
PPD is an acceptable ingredient in hair dyes that, when used correctly, do not come into contact with skin for prolonged periods of time. The use of PPD in cosmetics applied to the skin for prolonged periods of time poses a risk to the health and safety of the user. As such, "black henna" temporary tattoo ink and paste containing PPD is not considered safe.
Under Section 16 of the Food and Drugs Act, no person shall sell cosmetics that contain substances that may cause injury to the health of the user when the cosmetic is used according to the directions on the label or accompanying the cosmetic. Cosmetics containing PPD that are applied directly to the skin are not to be sold in Canada. This includes "black henna" temporary tattoos containing PPD, which are often sold and applied by artisans at markets, fairs and amusement parks.
Before receiving a temporary tattoo, ask the vendor to confirm that PPD is not being used in the ink or paste. Health Canada permits the use of natural henna and other safe dyes in cosmetics.
Check this link for more detail: Health Canada Caution