After the Boer War the British realized that South Africa was gonna forever be a battle between the Afrikaners and the British, which meant that either you would have four separate nations or one union.
Cape Colony started to enfranchise blacks by the mid 1800s, and even some Afrikaners believed in that. Afrikaners are largely divided into three groups - those who stayed in service of the Dutch (and later the British), those who stayed in contact with Europe and those who were too proud and African-souled, most of which went on the Trek.
Overall, I guess a liberal Cape Colony (people like Malan and Verwoerd would likely have been overruled as most of the Afrikaner unity came from the Transvaal and Orange Free State), a war-torn and likely troubled Natal (forget Durban being a major tourist place as it was in OTL and two very nationalist, racist nations in Transvaal and OFS.
Of course, by the 1950s the Cape Colony would likely have advanced like Canada/Australia/New Zealand did and be a first world nation, but Natal would be a mess and the Afrikaner republics would be economically and politically isolated.