There is no canonical in-game explanation concerning the origins of these murals, but it can reasonably be assumed that the residents of the nearby Triton Village might have painted them while their civilization was still thriving. The murals have been worn off of the walls below the waterline, presumably due to the ocean currents scouring the rocks and wiping away any traces of the art that might have been left.
The Blue Dome is also part of two post-game quests, in which Hayako asks the player for their help in researching the mysterious murals further. One of her requests is that the player take a picture of the Blue Dome murals for her.
Local Sea Life
The species that make their homes in the Blue Dome include a group of Blue-Spotted Ribbontail Rays. These stay near the seafloor, which is a flat, sandy, round area, and are loosely grouped together. Other creatures include the King Angelfish, a fish that sports a lovely deep-blue colour, and the Blue Tang, a fish that fits the Dome it lives in well, considering its stunning azure colouration.
"This domelike structure is found to the west of Ciceros Strait. Its forgotten mural is the remnant of an ancient civilization.
Visitors can expect to see shoals of bright blue fish such as blue tang and king angelfish."