It's the special sake recipe that they found in that book.I have no idea how to explain what gradation is, and I honestly think it might be a bad translation. The dictionary says it is a scale or a series of successive changes, stages, or degrees and gave the example:"within the woodpecker family, there is a gradation of drilling ability"
I'm wondering if "recipe," "technique," or "method" might be better words.
I think it means the blue colour would be a gradiation? Like darker at the bottom of the glass/bottle and lighter at the top (or vice versa)?
Based on the latest chapter, I think you're right.
They talk abt this blue graduation at least 100 times every chapt I've forgotten wht that acc is