One of the distinguishing characteristics of the highest level of Nubatama knives is the use of Black Steel. Indeed, the entire line of knives is named based on this fact - and the stones as well.
Nubatama is a word that means 'Black.' It is a subtle word, implying a beautiful black and also implying a characteristic of hardness. This word has ancient origins, being used in the earliest existing works of Japanese poetry. The Manyoshu a collection of poems compiled around 759 AD, includes several poems going back to 347 AD. You can find references using the word 'nubatama' in several tanka or short poems in this collection. There is also the association of the beautiful jet black hair of a beautiful woman. So these characteristics accurately describe the intentions of the knifemaker - a knife made of black steel, a deep sense and respect for ancient Japanese traditions, a sense of beauty and hardness describing the characteristics of this steel. A 'black beauty.'
The knives are made by a knifemaker whose family has been making knives for generations. While the details are a closely guarded secret, the steel itself is considerably more expensive than the blue or white steels and more difficult to work, yet produces a superb result. It is the highest quality of steel that the knifemaker uses. These knives are individually made. In addition, he also uses both white and blue steels, which are less expensive. Even here, his white steel forging processes are distinctive to optimize the steel's characteristics. He makes both single and double beveled knives, including ryobas or gyutos, santoku, yanagis, debas, usubas, and other custom designs. The double beveled knives are clad, which allows the steel to be tempered to a harder level. Edge retention is outstanding. This has been the opinion of chefs both here in the USA and in Kyoto. Touchups are easy and rarely needed compared to most other knives I've used.