Firstly lets dig into what henna is really made of...
Henna is a natural plant based dye that is made from the Lawsonia inermis plant species. The leaves are dried and grounded into a fine powder that is mixed with natural ingredients such as lemon juice, spring water, sugar and essential oils to create the henna paste.
After the design has been left on your skin for a sufficient amount of time, the paste is then removed and your stain will develop over the next 48 hours.
If your natural henna cone is left sitting out at room temperature it will release it's dye into the cone and that paste will not be effective in obtaining a good stain if applied to your skin.
Which is why natural henna cones are kept frozen until ready to use.
Over time companies started to manufacture chemically modified henna cones for sale to the public. These cones are unsafe because they are filled with preservatives and chemicals that allow these cones to sit at room temperature for extended periods of time. The stain obtained from these henna cones are chemical dye and not considered safe. These cones can cause severe damage and not recommended for use on your skin.
Another important fact that many people are not aware of is that henna stains are naturally a red/brown stain but henna is never black!!!! Black henna contains high levels of paraphenylenediamine (PPD) that can cause terrible burns when applied to the skin. It is extremely unsafe and should never be used.
If you'd like a darker henna stain find a henna artist that uses Jagua henna or Jagua gel for the design. Jagua juice is the extract from the Genipa Americana plant species, the extract naturally stains the skin with a dark blue/black color. Jagua henna is made from mixing the Jagua juice with the henna powder to obtain a naturally darker stain.
So please, when selecting your henna artist or buying henna cones make sure you're not getting chemical henna, be kind to yourself and invest in getting quality henna.