“Whom” contains the implied preposition “to” — as in “To [blank] must we blame”, which is obviously incorrect. “Who we must blame” is a reordering of “Who must we blame?” to indicate that the author claims to know who.
Sorry, but to rephrase the sentence, you definitely wouldn't say "We must blame he" . . . you would say "We must blame him"
I stand by my criticism. You don't even need the "to" to rephrase the sentence.
But I do except your apology.
But in the sentence “Who must we blame?” the subject is we, the verb is must blame and the direct object of the predicate is who. And Who is a nominative pronoun. The objective pronoun is whom. You would say “We must blame him”, you wouldn’t say “We must blame he.” He and who are nominative (used as subjects), whom and him are objective.