Mine is: Come at me bro
What if blast is actually Saitamas father...
It would line up with his age when he saved tatsumaki. Saitama was also 7 when it happened, and Blast was around 24-26 years old.
There is also a possibility Blast could be his brother, but they never met because Blast was always doing the hero work or slacking off, not caring about what's going on unless something really bad is happening. Both Saitama and Blast share these same characteristics, so it's a possibility.