Machetes and shovels? Really? Not even basic guns?
Where would they obtain said guns? The Apartheid government ensured that their were virtually no firearms legally in the hands of Black South Africans. Hell they sought to pretty strictly limit things like bladed weapons objects (Enforcing very strict laws in regards to public possession and carrying as so much as knives). In the short term the best that can be hoped for would be for Black militant groups managing to overrun some of the Apartheid Era South African Armed forces bases and capture their armories. If it was a few years earlier their would be nations that would conceivably try to arm Black South African militant groups (The Soviets, Cubans, East Germany and the like). But those countries all either no longer exist, are in the process of disbanding, or in the case of Cuba continuing to exist but cut off from Soviet subsidies meaning they are completely and totally broke and unable to engage in any sort of foreign adventurism. North Korea is too broke and undergoing it's own problems to subsidize anyone. Maybe some of the other African states would try and arm them but few of them are that well armed. And unless the militant groups can seize control of a functional port everything will have to be smuggled overland. And the Black Militant groups if it does end up evolving into an all out civil war are unlikely to have the hard currency to purchase arms on the international black market. Maybe if they manage to capture some of the mines and have some way to export the product of those mines they might have the ability to buy arms. The South African airforce would ensure that anyone trying to bring in arms by air dies quickly.
The balance of arms in South Africa at the time is overwhelmingly in favor of the White populace with the Armed Forces being of pretty decent quality and largely White (and at the time possesses a total of six tactical strength nuclear gravity bombs with the aircraft to deploy said weapons anywhere in South Africa). In terms of organizational comparisons the Apartheid Era National Party Government and Apartheid era armed services have any of the existing anti Apartheid militant groups beat by a wide mile. The ANC's militant wing was very very poorly armed and organized (to the point where they managed to kill something like less then a hundred people in a armed campaign lasting over half a century.) On the other hand the White South African government/military has no friends left abroad in 1991 and the best they can hope for is the Israeli's maybe covertly selling them arms.
Any civil war would be a incredibly violent affair with enormous casualties on both sides. If it lasts past a certain point there is a not inconsequential chance that it ends up involving the desperate Apartheid government using at least one of those nuclear weapons.
Thank god the transfer of power ended up being peaceful and the bloody hell of civil war never occurred.