The Blue are descended ancient Atlantean stock. When Zeus overthrew his father to take control of Olympus, his brother Poseidon became the lord of the seas. This mantle was stripped from an older god named Oceanus, son of Gaea.
In order to consolidate his power as an avatar for ideologue known as The Clear, Poseidon had his uncle imprisoned under the Pacific Ocean. He then made a pact with a number of Atlanteans with strong connection to The Clear. In exchange for a measure of his power and protection, they would forever ensure Oceanus remained trapped in his cage. The Atlanteans who accepted became The Blue and descended under the seas, well ahead of many of their cousins on mainland cities.
Under the crushing weight of the ocean, where the world is dark and cold, there are ancient paths to places where that which should not be free is chained and locked. These paths lead to such places at Tartarus, where Oceanus is imprisoned. The Blue guard these paths from their city of Xebel, just beyond a point where the worlds of man and god can reach.
When Ragnarok split the world, The Blue survived and maintained the prison's integrity with generations of magical sacrifice. Over time, this resulted in The Blue losing their Atlantean skill at magic. Only their close harmony with The Clear allowed them to retain their hydrokinesis and they became a fiercely warlike, clannish society.