Unfortunately, the Xbox Series X does not support Dolby Vision for 4K Blu-ray playback. According to various sources, including TechRadar and Forbes, the Xbox Series X supports Dolby Vision for streaming media apps and playing games, but it is not available for media playback via Blu-ray. This means that if you want to watch 4K Blu-ray movies with Dolby Vision, you will need to use a different player or device that supports Dolby Vision for 4K Blu-ray playback.
It's worth noting that the Xbox Series X does still support HDR10 for 4K Blu-ray playback, which provides a similar level of contrast and color accuracy as Dolby Vision. However, Dolby Vision offers additional features such as dynamic metadata that can adjust the brightness and color settings on a scene-by-scene basis, which can provide a more immersive viewing experience.
It's possible that Microsoft may add Dolby Vision support for 4K Blu-ray playback in the future via a software update, but there is no official word on this at the moment.