I saw this in two japanese movies (as I remember), and they were not from the same directors. First was Kagemusha, and the other one was Kwaidan. And they both used the same background style. It's kind of a paiting [that gave me an oniric sensation]. I don't know if this a technique that is commonly used in some japanese movies, or if they were just used by these two directors (one probably inspired the other).
You can call this "concept piece" where the matte painting effect is used. It has lately become bread and butter for the VFZ crew however the digital painting is under the art director's creative scope. This type of digital art can be created by using software as basic as photoshop and illustrator and later incorporated into the films through digital rendering or elevating into model type of layer art