Drat! I'll paste it below. It starts with a rather gross photo of a woman's hand with a henna-colored mehndi design that is entirely bubbled up in seeping blisters. (They don't mention black henna, but an additive called PPD- I don't know if that is what's used for the black)

"Henna tattoos are widely available and usually harmless. But certain kinds can cause a powerful allergic reaction.

Henna is a vegetable dye that can be brown, red or green, and it wears off in a matter of days. But to produce a darker color, some tattoo artists add a chemical called para-phenylenediamine, or PPD. The Food and Drug Administration says the only legal use for PPD is as a hair dye.

This photograph, published in the Aug. 6 issue of The New England Journal of Medicine, shows the blistered hands of a 19-year-old Kuwaiti woman who had a temporary tattoo applied at a wedding eight days earlier. She was treated with topical corticosteroids.

“The blisters lasted a week or so,” said Dr. Colby C. Evans, a Texas dermatologist and a co-author of the article. “It left behind a dark pigmentation that will take six months or more to fade.”

Is henna without PPD any safer? “There have been some reported cases of allergy to henna itself, but it’s rare,” Dr. Evans said. “Allergy to PPD is extremely common.”