First of all, I have a theory about the powers of Saitama. Maybe people like him i.e. who have god like powers always existed in the world for the protection of humanity. Blast could have been the person to hold gold like powers before Saitama. Saitama wished to have the power to kill any moster with only one punch so, he got that power. Similarly, Blast could have wished for some other power. It may have happened that, when Saitama got his powers, Blast lost his and Saitama took over as the new protector of humanity. After loosing his powers, Blast turned into a normal human being, unable to fight monsters anymore.
Now come the link between Blast and Mumen Rider (MR). MR must have been a kid when Blast was a hero, because he is of the same age as Saitama. Maybe MR was a orphan from childhood and Blast took care of him . When MR saw Blast loosing all his motivation to fight monsters after loosing his powers, MR joined the hero association just to prove him that one can save lives even without having any super powers. Now, during the Sea King fight, he first time met Saitama whom he initially though as a low level hero but, when he witnessed Saitama's power, it reminded him of Blast when he really cared for others. That's why he thanked Saitama at that noodle shop.