The lowest end interpretation would have the character in question scale to something along the lines of:
At least Subsonic (Moved faster than X subsonic character could see)
If the degree of the blitz is massive, you could even justify something like "At least Subsonic, likely higher/far higher"
If the character being blitzed is also close to baseline Subsonic+, you could easily justify an upscale.
IF the Subsonic character canonically has Superhuman/Subsonic perception or reaction speed, you could even have the blitz calced. But you need a direct statement for that, them just scaling to Subsonic off of another calc won't work since it's calc stacking.
So, basically, it's a case by case thing.