Since spotting Rihanna’s tribal-meets-henna-inspired tattoo at   Balmain,   Stella McCartney,   Givenchy, and pretty much everywhere else in Paris this week—not to mention on the March cover of  Vogue—I’ve been mesmerized by the traditional Indian pattern.   So I did what any true beauty sleuth would do: Following a brief Internet search, and a few investigative Frenglish phone calls, I tracked down the nearest tatouage au henné  establishment.