Joe Offer's comparison of Máire Ní Scolaí with Paul Robeson is apt, I would add contralto Marian Anderson as well. Ms. Anderson, within this rather dignified, restrained convention of singing, actually did switch between spirituals and the German Lied with distinction. And she sang Schubert's "Der Tod und das Mädchen" as if she owned it.

https://www.youtube.com/watch?v=FbH1w_c_rxo