“Never apologize: it’s a sign of weakness.” (John Wayne)
“Whom must we blame?” “We must blame him!” “Whom” and “him” are both objective-case pronouns, even though “whom” has been relocated to the beginning.
If you want to think of the pronoun as the object of a preposition, you could phrase the sentences, “To whom can we assign blame?” “We can assign blame to him.” However, this is not necessary for us to need an objective, rather than subjective, pronoun.
(This grammar clarification has been offered in memory of “Nina0113,” who loved a good compound-complex sentence almost as much as she loved a needy cat.)
You're still wrong, but life must go on. Interesting chat . . . but I won't budge.
I defer to the expert! Thank you, Tax-chick!