Actually:

• The blockchain that you should trust is the valid blockhain that you can get that has the most proof of work.
• A blockchain is valid if and only if the software you are running says that it is valid.
• "The bitcoin blockchain" is the blockchain that you call "the bitcoin blockchain".

The question "in a persistent fork of the blockchain, which branch will be the real bitcoin?" is basically a religious question.  You should think about it as you would think of "in a schism of my Church, which side will be the True Church?".