I believe the term is "appositive." It + was + they + who were to praise. a) We say that "who were to praise" is IN APPOSITION WITH "it." (That is, "who were to praise" explains the "it.") b) Native speakers do not accept "Who were to praise was they." c) So we start the sentence with "it." "It" is a substitute for the words "Who were to praise." d) It (who were to praise) + was + they. IMPORTANT: Some books have a different analysis: It = subject was = linking verb they = subjective complement who were to blame = adjective clause that modifies (refers) to "they." You will have to ask your teacher which analysis she or he expects.

http://www.ldoceonline.com/dictionary/apposition This sentence does not fit the usual definition of an appostion. I suppose 'they' and 'who were to praise' are both describing the same thing.

I believe the term is "appositive." It + was + they + who were to praise. a) We say that "who were to praise" is IN APPOSITION WITH "it." (That is, "who were to praise" explains the "it.") b) Native speakers do not accept "Who were to praise was they." c) So we start the sentence with "it." "It" is a substitute for the words "Who were to praise." d) It (who were to praise) + was + they. IMPORTANT: Some books have a different analysis: It = subject was = linking verb they = subjective complement who were to blame = adjective clause that modifies (refers) to "they." You will have to ask your teacher which analysis she or he expects.