Where is All Blue located?
There is no canon or non-canon answer from the One Piece universe.  Sanji referred to a lake on Fishman island as his All Blue but that was not serious (I think). Until Oda says, we don't know.

We could speculate it could be reverse mountain, or a bay at Raftel, or a cavern deep underground, or Vegapunk's research aquarium on the moon, or an alternative dimension (an island ate a devil fruit?), or ..... The point is that Oda is very creative.  It could be many things as a result.