i think the above poster is referring to ppd, mentioned here

http://www.ncbi.nlm.nih.gov/pubmed/17163917

and at first i thought he meant making contact prints with it, but i now think he means making skin contact with the henna gives lifelong allergies (as mentioned in the link above). And from what i understand he means the 'black henna' which you should look up, and see the nasty reactions people have.

Anyway, interesting question, thought i could learn something from it, and possibly suggest getting some coal and mixing that in with the indigo? would that work?... or would it just smudge? keep trying and hope you find something that works.