Is 'to' in this sentence a 'preposition'?
There is no point having a go at me. The witch is TO blame.

Question

Answer

"Blame" is a verb in infinitive in this sentence, thus "to" is not a preposition. "To" is a "function word" that indicates the infinitive form of the verb.

Answer

"The witch" is the subject. "is" is a linking verb that describes the state of the witch. "TO blame" is an infinitive phrase that acts as an adjective.

So, "TO" is not a preposition.