The best I can tell from my stone, it is not a Tsushima mountain and it is not an ocean black. It is very hard and very smooth. Although the razor finish looks like a mirror finish to the naked eye, if it is examined at 30 - 60x, there is some haziness, just as other have reported with a JNAT.
I can get a true mirror polish with my Naniwa 12K, but the edge is tends to be somewhat harsh with some steels. That is why I tried the Tsushima blue. The edge might not be as sharp, but it is more smooth.
I am anxious to see what kind of results others who have indicated they plan to purchase the stone will be able to achieve. This stone is not going to replace Eschers, Coticules, and Nakayamas, but for those who do not want to pay the exorbitant asking prices for these stones, the ocean blue may be an alternative. Yes, I could have purchased a Coticule bout for a similar price, but I would rather have a rectangular stone.