The deep, green-leaning blue of cloth dipped and re-dipped in fermented indigo. When Edo sumptuary laws barred commoners from bright silk, dyers answered with a hundred named blues — this is the one Meiji-era visitors called "Japan blue."
Indigo arrived with continental trade before the Nara period, but it became a civilization in Edo: whole river districts fermented sukumo leaf-balls in sunken vats, and everything from firemen's coats to farmers' momohiki came out of them. The chemist Robert Atkinson, visiting in 1874, was so struck by the streets' uniform blue that he coined the phrase 'Japan blue' — a century before denim made the same color the world's workwear.
The Engishiki — the court regulations completed in 927 — records exactly what the palace dyehouse was allotted to dye this color. Quantities are per bolt of cloth; units are left in the original (1 kin ≈ 600 g, 1 shō ≈ 0.7 L in later standards — Heian equivalents are debated).
The Engishiki also gives scaled quantities for: 1 bolt of plain silk · 1 bolt of silk for resist-dyeing · 1 skein of silk thread.
深藍色糸一絇、藍一囲小半、黄蘗十四両、薪廿斤、
中藍色糸一絇、藍一囲、黄蘗十四両、薪廿斤、
浅藍色綾一疋、藍半囲、黄蘗八両、帛一疋、藍半囲、黄蘗八両、纈帛一疋、藍半囲、黄蘗八両、糸一絇、藍小半囲、黄蘗八両、
白藍色糸一絇、藍小半囲、黄蘗七両、
Text: 『デジタル延喜式』校訂文（縫殿寮）, National Museum of Japanese History · 1900 printed edition (NDL)
Complementary and triadic companions, chosen from real traditional colors — not computed values.
藍染め 藍甕 — what the name literally points to.
