The only references to "pelagic" or any word like it that I had heard of before seeing this image were references to the open ocean (e.g., "pelagic biome" that extends from the open, non-coastal, ocean surface to however far down the bottom may be, up to 10.9 km). The name Pelagia means "of the sea" according to Think Baby Names.
Her genealogy can be seen here.
This does not look like everyday dress; she would be noticed if she wore it on the street.
Keywords: 1793, Vigée-Lebrun, Pelagia Potocka, Potocki family, Sapieha family, Countess, long curly coiffure, turban, single shoulder neckline, waist band, natural high neckline, close skirt, bracelets, wrap, theatrical dress