EX: He is to blame.
be to blame for is synonymous with be at fault for. It means, (in a) state of being at fault, and it's an idiomatic expression:
He is to be given the blame (by us) for what happened.
"to" is an infinitive marker. It marks the base verb "be", and "blame" is a noun; note the article "the". If we omit the passive verb "be given", the resulting form "to" + "blame" is an infinitive verb in form, and functions as an adjective:
Q: What is he?
A: He is (the person) to blame. "to blame" modifies "He"
We blame him. (active voice)
He is to be blamed. (passive voice; the accuser is left unsaid)
He is to blame (active voice, with the accuser left unsaid)