Moroccan ladies desperate to paint henna designs on foreigners' hands and feet seem to be everywhere in the Medina, with the biggest concentration in el Djemaa el Fna.
There are two types: natural brown henna and a chemical dye known as black henna. The latter lasts longer, but it is dangerous, as it can cause bad skin reactions and even cancer.
The henna artists in Djemaa el Fna usually operate in groups of three and can sometimes forcibly grab a foreigner's hand and start squirting henna on it. Try to avoid them. If you want a henna design, get chatting to people in the souks and ask them to recommend someone. They will then fetch a traditional henna artist, who will charge far less then the ones in the square.
So for my last two trips I have gone back to the same lady, she charges what equals only about £5.00, and does a very good henna tatoo. I'd advise you to wait and have it done at the end of your holiday.
Some people will tell you the henna is black, but later that evening when the henna falls off you will soon realise that it is weal brown henna and will mark your hand for a couple of days max....it might last for two swimming pool dips, and doesn't look very nice on your hand!
However the lady I found is lovely! This year, I looked for her and eventually found her, I arranged for my hotel to write her a letter in Arabic. She couldn't read but people on Stall 22 read it to her, she was really pleased because the letter was praising her work and told her how pleased I was last year.
Myblack henna tatoo lasted approx 30 days, you have to be careful with black henna, it can be very powerful and damage/burn your skin. However this ladies work is very good.
"Henna ladies" (as I like to call them) are ubiquitous in Morocco. Often they will aggressively grab your hand in an effort to start the hennaing process before you can say "no" or strike a price. I found that the best way to deal with this is to have your hands hennaed at the very first opportunity (either back home or when you first land in Morocco). When a Henna Lady grabs your hand and sees that someone has beaten her to you, she'll back off immediatel ... it's easier than saying no over & over again.