The ranking system of the Hero Association in One Punch Man ranks heroes not only according to their strengths but also according to charm, mystery, and public perception. S-class heroes are the very uppermost stratum of acknowledged power in a world otherwise chronically threatened by calamitous disasters. Some carry on with their duties by protecting and rescuing; the actions of others maintain the enigma of secrecy, either by their natures or by the little the Association has chosen to tell. Such characters, of course, become the subject of much gossip: sporadic sightings and quiet reputations build up a mythos that can easily eclipse whatever evidence remains of their real deeds of heroism.
Or the top-ranked S-Class hero in One Punch Man has made quite a reputation for himself. His very rare appearances in both the manga and the webcomic paint a picture of a being with immense, possibly incomprehensibly immense, powers. More on a level of dimensional surveillance and combat, untethered to earthly concerns, this points to a being of cosmic responsibility on a universal scale.
Whilst there is no doubt that Blast holds the title of strongest being under the Hero Association, carrying the weight of a world under threat, the inexplicability of the extent of his powers and possible responsibilities remains very much there.

He is said to exist, in part, because of admiration and curiosity from other S-Class heroes. Tatsumaki, Bang, and even Genos acknowledge his legendary status, albeit with little knowledge about the man himself. This paltriness of information grants him a mystery and powerful aura—a guardian over threats that transcend the lower-tier problems of the world, still under wraps.
When Blast decides to show himself, the feats on display separate him from any known hero or villain. His particular mode of dimensional travel suggests technology or powers not known to any hero or villain.
He interacts casually with mysterious entities, collects powerful cubes associated with godlike beings, and is privy to a lot of information concerning a wide-ranging multiversal conflict. The argument can be made that Blast is strong not just in combats of a more traditional variety but is probably also a cosmic protector against forces that hold sway upon reality itself, while the greatest majority remains ever ignorant of them.

Blast is very aloof and out of touch with the rest of the Hero Association. He was not seen intervening in cases like that of threats set forth from the rise of the Monster Association or Garou's rampage, leaving doubts about whatever criteria he follows to act.
Speculation goes on about him presumably focusing entirely on multidimensional threats, confiding the rest to be dealt with by his subordinates. Such detachment would not alienate irresponsible, yet perhaps rather a division of labor on the scale. This absence of universally accepted crises irks fans and characters alike, but in doing so, it enhances the notion of Blast's dealing with an entirely different class of threat.
Blast’s Role in the One Punch Man Manga’s Current Storyline
Blast may not have been seen all that much recently in the One Punch Man manga arcs, but his presence has been felt. His ups and downs with Saitama and Flashy Flash indicate that he is watching the anomalies in space-time and knows that certain individuals are significant to keeping things balanced. Until now, the One Punch Man manga has not mentioned Blast's full-fledged war with the adversary, but his warnings about an upcoming god-level threat suggest that a real cutthroat clash may soon see the light, where he would perhaps expose his full strength.
Meanwhile, the One Punch Man anime has not gotten to the point where Blast makes an actual appearance, though they mention his name and legend. Future seasons would, of course, likely expand on this role, especially as the story gains focus on more abstract and universal dangers. If the anime continues to adapt to the current manga, Blast's character could add another important link between Earth-bound heroes and more distant cosmic stakes, which threaten existence at the moment.
There is no detailed post-epilogue or definitive ending for Blast's arc, as the One Punch Man manga is still ongoing. His entrance, however, has greatly widened the scope of the series to transform One Punch Man into not just an action-based anime but a more multi-layered and broader narrative. The future potential interplay may bring battles that happen beyond planets, thus adding stakes that exceed all current known threats.
In many ways, Blast is made out to be one of the most powerful and important characters in One Punch Man, not only in terms of strength but also in terms of cosmic importance. What remains mysterious about him is that much is revealed about him, as mentioned previously, and thus positions him as a force that is unlike any other. As the series progresses, both in manga and the ultimately expected anime adaptations, much will be told about him and how essential he is for the protection of humanity and reality itself.