It strikes one as a mild expletive (4)
I believe the answer is:
'it strikes one as a mild expletive' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for blow that I've seen before include "Powerful stroke; completely bungle (an opportunity)" , "Setback" , "haymaker, perhaps" , "Shock; sound (a horn)" , "Hit - disappointment" .)