Why is All Blue not part of the 8 Seas? While exploring this site, I came across this particular question: What are the 9 mountains and 8 seas? 

In the answers however, I did not see anyone mention All Blue. So my question is why is All Blue not a part of the 8 Seas that Zoro mentions?
Because All Blue is not a sea in itself, but the point of union of all 4 great seas where every kind of fish of every sea can be found.

Source: One Piece wiki