Hero Association won't make an imaginary hero as their stop gap plan, in a God level threat case a false hope'll generate no practical value.

So the by elimination we can assure Blast isn't any hero, blast is not Pig God, Blast is not imaginary, despite that there is no record that Blast has eliminated this threat or that demon. Let alnoe there isn't even a record of a God Level threat before Boros. So when we eliminate the impossible, whatever remains however improbable has to be the truth.

Blast is an alternate personality of Saitama. Saitama has already lost his feelings, he forgets names. So Blast is the hero which Saitama strives to become. 1)Independence in Working 2) Taking care of God level threats 3)No records so that nobody disturbs him 4) Ending all fights in one punch.

Special points( Boros before dying said, "You never used your true power.") so Saitama still can limimt his power, so the alternete personality with no control on his limitless powers, inside saitama's psyche.