Black henna is really indigo, a blue dye made by fermenting the indigo plant, originally from India. For centuries, indigo was very rare and expensive, which is why only the rich could afford purple clothes dyed with this most exclusive pigment.
Indigo, the plant dye from Indigofera species and grown in India, is identical with woad - yes, the blue body paint from Braveheart - which is obtained from a quite common European plant, Isatis tinctoria. Not only was woad not particularly rare - it was traded in bulk - it was the only stable blue dye available. It was the second most important import into Southampton, England in the 15th century.
What the author has got it confused with is Tyrian purple. This was a purple, not blue, dye which used to be produced from Mediterranean shellfish, typically Murex trunculus and Murex brandaris, in phenomenally tiny quantities - it was said that it took 12,000 shellfish to dye a woman's dress. This is the dye that provided the stripe of rank on a Roman toga; as you can see, only the rich could afford that quantity of the dye.
The development of the synthetic dye mauveine by Henry Perkin in 1856, which was not a synthetic indigo but the first aniline or coal-tar dye, was what led to the shocked reaction at servants wearing 'royal' purple.
*grin* Now that I've bored you sufficiently, I think it's time to rinse my hair...