'Dark (kuro) sake'. 'Ki' is the old name for sake (rice wine), and sake is known on ritual occasions as (o)mi-ki. Shiroki and kuroki (light and dark sake) are special kinds of sake offered as shinsen at the niinamesai (autumn festival), including those occasions when the niinamesai is a daijosai or accession ceremony for the new emperor. Dark and light sake have also traditionally been interpreted as refined and unrefined sake, however instructions for making these offerings are found in the Engi-Shiki, where light sake is natural sake and dark sake is made by mixing ashes of kusagi (a kind of arrowroot).
A Popular Dictionary of Shinto. Brian Bocking.