Yeah, niger seed (Guizotia abyssinica) is not from a thistle; it’s related to sunflowers.
The name might become confused with Nigella sativa. That’s right, the famous English TV cooking lady has the same name as a spice. Nigella sativa is used in Indian, Afghan, and Persian cuisine. In India, the seeds are called kalaunji (derived from kala meaning ‘black’). In Arabic, it’s called “the Black Seed (al-habbah al-sawdâ’)” and is considered a good herbal medicine. Indian groceries sometimes sell it with the name “onion seed” which is ridiculous since the plant is not related to onion. Horticulturists grow Nigella sativa as an ornamental plant and call it “Love-in-a-Mist.”
In Latin, nigella means ‘the little black one’ (feminine), diminutive of niger.
However, it has been suggested that the name of the Niger River is not originally derived from the Latin word for ‘black’, but comes from a phrase in the Tuareg language, one of the Berber languages indigenous to North Africa and the Sahara, in which n’eghirren means ‘flowing water’.