I would be careful, though. "Black" henna can contain PPD, or
para-phenylenediamine, an ingredient in hair dye that can cause
scarring on the skin. However, it is safe to get an indigo tatoo, which
is sometimes marketed as "black henna" because it will dye hair black.
Indigo on skin leaves a blue or black design (depending on how many
times you apply it). Indigo is safe, PPD is not. Just be careful!