All Blue is the sea of legend in the world of "One Piece," the popular manga and anime created by Eiichiro Oda. The legend has it that this is a special ocean where the four seas in the One Piece world are combined: North Blue, South Blue, East Blue, and West Blue. It's an integral part of the dream of one of the main protagonists, Sanji, who happens to be a chef and a pirate. Sanji would look for All Blue with such eagerness because of his notion that this sea contains the most varieties of fish from all four seas in one place, which would just be perfect for a culinary mastermind like him.
Throughout the series, however, the location of All Blue is not known, though it is speculated to be somewhere in the Grand Line. This perilous, mystical equatorial sea circles the globe. All Blue is a search for rare ingredients, but symbolically, it means something much deeper—the longing for an ideal place where all the seas of the world meet. So, it is unity and perfection, something which is very rarely in the fractured and battle-scarred world that is "One Piece."
Although the All Blue is central to Sanji's backstory and motivations, it has still not appeared directly in the series. Its mystique lives on, driving both fan theories and actual plot progress in "One Piece." Sanji's resolve to find All Blue shows that he does not lose hope and, at the same time, never gives up—two very important qualities in this narrative.
The All Blue functions as a metaphor to point out the place where all differences would be ignored, which means that it is a place where diversity in the world can live in harmony. This is something ideal that even beyond the series tells the audience that somehow, most of them find Sanji's dream compelling to come true.