Para-phenylenediamine is the chemical basis for most 'permanent' hair dyes, regardless of their color. The exceptions are temporary dyes that are fully removed with the next washing (eg. your kid wants blue hair for the day), semipermanent wash in colors (most famously the old 'Grecian Formula') and henna used alone. The temporary dyes are free of PPD; their only risk is to one's fashion sense. The semipermanent ones classically worked by gradually adding lead acetate to your hair. Used for decades and blessed as safe by the FDA as recently as 1980, lead acetate has been banned by the Trump FDA in the last month or so. Its former products have switched to other chemicals, most often bismuth citrate, but a quick internet search suggests the alternatives don't work as well. This dermatologist just learned of that ban today. Henna is a plant based 'natural' product and it permanently colors the hair without washing out. However, by itself, it only produces one color, famously the trademark color of that natural blond, Lucille Ball!
PPD is one of the more common causes of allergic contact dermatitis and can produce strong reactions. It's only a problem as a liquid, until it's set. Once set and dyed the colored hair doesn't cause allergic reactions. Allergic contact dermatitis to henna has been reported, but is rare. Dying your hair with henna works well if you want to look like Lucy. Henna has long been used to temporarily stain the skin (a 'temporary tattoo'). Used alone it gives a reddish brown color, which isn't very dark and wears off fairly soon. Adulterating 'henna' with PPD produces a darker color which lasts longer, which those wanting a 'temporary tattoo' prefer... unless they become allergic and this happens. 'Dark Henna' really isn't a different form of henna, it's henna PLUS dark (PPD) and in many cases most of color is from PPD. As PPD allergies are common, reactions like this are common to dermatologists. They're normally treated like similar amounts of poison ivy rashes, combined with avoiding PPD in the future. Which for many eventually means embracing their eventually grey hair.