WARNING: The following contains spoilers for One-Punch Man Chapter 137, by ONE, Yusuke Murata, John Werry and James Gaubatz, now available in English from Viz Media.
While he's only appeared in flashbacks and silhouettes thus far, top-ranked S-Class hero Blast finally appears in the present in the latest chapter of One-Punch Man. In the process, he meets Saitama, potentially saving the incredibly powerful hero from the same malevolent force that gave Psychos her powers.
At the start of One-Punch Man Chapter 137, Saitama, Flashy Flash and their monster companion, Oculette, are still underground as the fighting between Psychos and the rest of the manga's heroes rages on above. Looking through the hole opened up earlier in the manga, the trio hear a disembodied voice that tells them, "Place your hand on that box and prag. A deserving vessel will obtain power. But the undeserving shall be collected." Nearby, there's a black cube impressed deeply into the floor.
Flash Flash remarks that what they're hearing must be telepathy, and Saitama vows to fight the voice, believing it's actually the true leader of the monsters. However, before they can act, a black sphere appears out of which steps Blast. The hero's armor is covered in scratches, and he has a long scar across his face, which contrasts greatly with his appearance in past chapters of One-Punch Man. Blast is happy they didn't accept the deal and grabs the cube, which he explains "is a kind of transmitter. Touching it warps space so you can communicate with it." Blast says that these types of transmitters are all over and the he collects them "for fun."
After Flashy Flash confirms that the hero in front of him is indeed Blast, the top-ranked S-Class hero questions why they're working with Oculette, though the hero takes Saitama's explanation that she's useful at face value. From there, Blast prepares to leave, saying he'll eventually meet Saitama and Flashy Flash again. The powerful hero then pushes his two gauntlets together, and they form an infinity sign that in turn creates another black sphere in which he leaves.
While Blast still remains one of One-Punch Man's most mysterious characters, this meeting with Saitama and Flashy Flash show that he's very aware of the divine power that's empowering Psychos. At this time, it's not totally clear just why Blast is collecting these black cubes, so there's a chance he may not be as heroic as he first seems. Still, had Saitama, Flashy Flash or Oculette followed the disembodied voice's directions, they may have been forever transformed into something else, much like Psychos. Had this happened, Saitama may have had to fight this divine power directly or face an ally enhanced to the level of Psychos, which could have resulted in catastrophe. So for now, Blast seems to have saved the Earth from another potentially deadly battle, even if that might have meant a new challenge for Saitama.
One-Punch Man Chapter 136 does not have a release date at this time.