I've just watched the anime, in there Blast doesnt appear, but I believe he was mentioned when the heros gathered to fight the aliens. Supposedly he's the #1 hero of the hero association but he never appears. Does he appear in the One Punch Manga or at least in the webseries?
Currently, Blast has only made an appearance in flashbacks or as part of exposition.
In one of the last few chapters of the webcomic, Tatsumaki had a flashback where she encountered Blast.
In the Manga, shortly after the Elder Cendepede fight started, Phoenix man was enumerating the heroes he believed could possible win against Elder Centepede, and mentioned Blast. The back of Blast's head was shown then.