I was flying to Tokyo and noticed in the VRF map an area surrounded with a blue border. What is this? [Microsoft Flight Simulator Screenshot 2022.10.03 - 14.57.58.16]

It´s most probably a nature park near Shikama. Some dams there and waterfalls. See SkyVector and Google maps. [RJR-127_SkyVector] [RJR-127_Google]