"Chrysophanic acid and Emodin are two anthraquinoid dyes that are present in the roots of Rhubarb. As a dye (Chrsophanic Acid) can be used to make yellow, orange and red shades with suitable mordants and is sometimes used as a dye for hair, since Chrysophanic Acid will bind to the keratin in hair......Physcion or Parietin is another anthraquinone found in Rhubarb. It has anti-tumour and anti-fungal properties and is orange-yellow in colour."
What this means is that this stuff is absolutely fantastic for hair health, regardless of whether you're blonde or not. On blonde and lighter hair colours Rhubarb Root Hair Dye will enhance your natural colour tones; on darker hair colour tones a plain water mix, or a couple of tablespoons thrown into your henna, means that any colour impact wil be negligible - a bit like cassia obovata - to the extent that you won't notice any colour, but you will notice the conditioning effects.