The name means “Black cards”, which is especially true of you look at the suit of batons. The horse's legs can be recognised on the Cavaliers.
The court cards have become abstract forms with almost no visible indication of what suit they belong to. The idea was to disguise their appearance after Mekuri games such as Unsun Karuta and Tenshô Karuta were banned by the authorities, especially if played with foreign cards.
In general terms, Japanese playing cards are of two types: 'Awase' or 'matching pairs' cards and Portuguese or Spanish-derived 'Dragon' type cards. These have dragons on the aces and are believed to have originated in Spain, Italy or Portugal.