According to the Study Commission on U.S. Policy toward Southern Africa, the White population in 1980 was 16.7% of the population; they were largely concentrated in the cities of the East (Largely Afrikaner/Boer there), and in the Cape Province (Largely the English speaking Whites). This means that in the opening phases of a conflict, the SADF will be able to quickly gain control over the vast majority the largest province in the Republic, as well as its economically vital cities. Long term I can definitely said cities being evacuated by SADF, with the Whites, Coloureds, and Asians fleeing to the Cape. Don't know how true it is, but I have been told second hand that the Black neighborhoods in cities by the 1980s were largely designed so that, in case of an uprising, their Air Force could easily bomb them.
Lastly, to speak of a unified Black identity/faction in a Civil War is a nonstarter. As others have alluded to, there was particularly savage direct fighting between the various nationalities, in particular between the Zulu-aligned Inkatha Party and the ANC. In fact, one of the things Mandela demanded in the lead-up to the end of Apartheid was the end of support the the Nationalist Party was still giving to the Zulus to undermine the ANC. I fully expect that, should a Civil War break out in South Africa, you're just as likely to see the Blacks fighting each other as they are the Whites. Indeed, such divides are ongoing to this day as the
Marikana Killings show.
After the 1983 Reforms, Indians pretty firmly slid into the Nationalist Party/Pro-Apartheid camp. In the lead up to the 1994 elections, Black on Asian provocations in particular were worrying for them,
as contemporary reporting by the New York Times shows: