anthocyanin, a powerful antioxidant, anti-inflammatory plant pigment that colors rice black-purple-reddish
Just like it does with blueberries, sweet potato and red cabbage.
anthocyanin is water-solvable
Anthocyanin dissolves in water and (alkaline) water will color black. Simple as that.
Soaking rice overnight results in up to 15-20% of loss. At home we don't soak our rice and many of our customers don't do either. The longer one soaks, the more whitish the grain becomes.
In nature pigment is always located on the outer layers. This is no different for a rice grain.
The core of the grain, the endosperm, contains the brown-whitish macro-nutrients (carbs, protein).
anthocyanin is stored in the bran, a layer on the outside of the grain
to verify authenticity, do the vinegar test
The vinegar test is the golden standard and anyone can do it.
The principle is this: anthocyanin acts as a pH indicator. Dissolve black rice in vinegar, an acid solvent and the solution colors deep red. That's what you want to see.