I had heard a speculation that "the black boy frae yont Nyanga" actually referred to Nelson Mandela and the song was written at the time of his anti-apartheid activities (Nyanga was prominent in this) - which eventually led to his imprisonment in 1962. Nyanga is one of the townships around Cape Town, then and now "one of the most dangerous places in the world".
However, according to other sources, the song was NOT about Mandela, but Hamish Henderson did write about him in the "men of Rivonia" - referring to Mandela's trial.