Avoid hennas advertised as "black henna" like the plague. They can contain chemicals that can horribly burn your skin. More info on it
is here, although I will warn you that the pictures of what can happen are disgusting.
Thank you! I've been warning peopel for a very long time about the dangers of "black henna". Basically, it's concerntrated black hair dye.
You want to purchase the henna powder/paste from a reputable person, or simply get it done by a reputable artist.
The darkness of the stain will depend on the crop and where the henna is from (some hennas are more brown, others more orange, others more red, etc). To help darken the stain, Keep the henna oon as long as possible (usually overnight is good), have it sealed (some do a mix of sugar and lemon juice, others use thick spray-on hair gel, others use NewSkin (liquid bandage), etc etc etc. You can also wrap it once it's been sealed to keep more body heat (which helps too).
It all depends on many, many factors! If you need any info, let me know and I can send you links to some informative sites.
I'll be getting henna done at the end of Ramadan (I'm fasting, so I figured I could get the henna done). Usually I do my own, but for once, I want some henna on BOTH hands, not just my left hand. LOL!