I'm wondering if she needed to invoke some authorities for her works (perhaps partly due to cultural sexism and her own lack of credentials) and so a class of beings reminiscent of saints, angels, bodhisattvas--beings "higher" than human but not quite divine--was needed. If I recall correctly, there was a precedent in the hermetic tradition in figures like Hermes Trismegistus; Blavatsky may have blended some such with the versions of Buddhism and Vedantism that she was promoting, along with her own, er, unique contributions.