A sentence from an ACT English:
His 1984 album Buscando América is a call for reform, which Blades believed would be possible only if Panama were freed from then dictator Manuel Noriega's oppressive regime. Blades believed this
I think the sentence should've written as " call for reform, which, Blades believed, would be possible" in which " Blades believed" serves as a parenthesis and the antecedent "reform" is used as the subject in the relative clause.
However, if let the sentence as it is, I cannot exactly tell what does the antecedent "reform" serve in the relative clause, subject in the object clause following "believed＂？ I think it cannot be used that way.