Sometimes there is conflicting information about the origins of some varieties out there, and it can get a bit confusing to find the truth. Sometimes even two published books from the same publisher will state conflicting information about the origin of a variety.
In any case, here's the Korean Neo association's profile on Kurokongo:
흑휘(黑輝) 흑금강(黑金剛)
It says:
"Bean leaf Amami fuuran seedling mutation. While the shape is similar to Tamakongo, it is slightly smaller, and produces a lot of anthocyanin pigmentation, to the extent of turning black if grown in high light. This variety goes by the name Kokki as well as Kurokongo. Flowers face upwards. Leaves are wide and deep green, with narrow spacing between leaves. Mud stem, mud root, tsuke is a mix of straight and wavy."