Aoji

Blue ground. Sumi that has not yet emerged to the surface of the body. It is also called Soko Zumi (Soko means 'bottom'). It is primarily seen in Shiroji and appears to be a transparent blue. It indicates unfinished Sumi. This term is only used for varieties with Sumi such as Taisho Sanshoku. When the Aoji emerges the Koi will be complete.

(Courtesy of the excellent Book Kokugyo Vol II)

Next: Bongiri