I have researched herbs used in diabetes treatment formulations in Ayurveda and Traditional Chinese Medicine. I did not see any major reference on the use of Nigella sativa (common name- black seed) in treating diabetes.
But according to the Memorial Sloan Kettering Cancer Center, various scientific studies indicate that molecules from the plant have immune-modulating, antioxidant, antiparasitic, and hepatoprotective properties. This reference further states that the seed of black seed may be useful in treating asthma, hypertension, rheumatoid arthritis, dyspepsia, diabetes, dermatitis, and even cancer.
The fruit extracts have been tested in diabetic complications and aging (Ahmed et al., 2014).
Beyond these one odd references, the scientific literature on the plant possessing anti-diabetic properties is tenuous.
In Arabo-Islamic culture, black cumin has been prescribed for various ailments, including fever, asthma, chronic headaches, diabetes, digestion, back pain, infections, and rheumatism. In Arabic culture, black cumin is known as Habbatul barakah, the seed of blessing. In fact, since its rise in popularity in the seventh century, it is still regarded as important family medicine and the oil that is most often used medicinally.
Probably, the claim on black seed possessing properties to treat diabetes comes from the above-mentioned claim. As stated earlier, these claims are not supported by substantial scientific support.

Additional reading:

  1. Holy Herbs: Modern Connections to Ancient Plants
  2. Natural Solutions for Diabetes

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