Is it correct to say 'It was they who were to blame'?

Yes, the sentence is correct.

The pronoun 'they' is a subjective personal pronoun. In the example sentence, the pronoun 'they' is functioning as a subject complement following a linking verb (It = they). A pronoun that follows a linking verb is always the subjective form.

A linking verb acts as an equals sign, the object is a form of the subject (Mary is my sister. Mary=sister); or the subject becomes the object (Mary's feet got wet. feet->wet).

A subject complement takes the place of a direct object following a linking verb and restates the subject of the sentence.

The subject complements are:

• a predicate nominative is a noun or a pronoun following a linking verb;
• a predicate adjective is an adjective following a linking verb.