Bizenyaki ‘Blue’ by Kobashi Toshimitsu of Mu Gen An.
If you’re interested in following the Mu Gen An studio, here is a link to their instagram account (https://www.instagram.com/mugenanbizenyaki/)
Below is a video detail of the cup for sale:

This ‘Aoi’ or ‘blue’ style tea cup is an unusual type of Bizen pottery (Bizenyaki) due to how the blue color forms (reduction — absolute lack of oxygen) in the kiln. The reduction situation in the kiln is something the potter can influence through the use of a sealed vessel called a saggar. By putting combustible items in the saggar along with the cups they intend to turn blue, oxygen can be removed from the cups immediate environment during kilning.

While perhaps technically not difficult, it has to be made with purpose. ‘Blue Bizen’ is not a normal sight in the world of tea ware, it is reserved more for use in the sake and alcohol world.

I usually buy cups from Toshi annually and they arrive in October/November, they are entirely hand made and one of a kind.

This is the last blue cup I have from the 2021 batch. I would love for you to see them in person if you’re interested.

Feel free to read this article about Bizen pottery here:
https://www.sommerier.com/?p=2150