You're asking for a POD that would prevent the United States' from becoming independent after their victory at Yorktown, in essence.
Belgium's acquisition of the Congo was the event that paved the way to the division of Africa. However, all those factors were already in place and already moving. The French and British in particular had already started their games and others were eager to throw their hat in the ring. You would need a massive and devastating war to occur mid-process. Probably between Britain and France as the one nation that really brought them together (Germany) has been prevented from forming.
That said, there are plenty of African countries that can survive this relatively unscathed or even becoming great powers. The Zulu are NOT one of them. Shaka believed writing was worthless, guns were slow, and that european tactics and technology were inferior to his own. For a time this was right, but the moment gun technology improved he was doomed, as were his people and their independence. West Africa is filled with powerful states that rose and fell, Sokoto comes to mind. Morocco and Egypt, for North Africa. Madagascar was pretty capable of asserting its independence. Ethiopia is an obvious one. So you can have strong independent African countries there.
Those were the candidates I was thinking of. Some other candidates I was thinking of was maybe trying to keep some of the other Fulani states, Buganda, Kingdom of Kongo, Zanzibar, Asante Union, Bornu Empire, Kingdom of Burundi, Kingdom of Rwanda, Liberia, and Maryland.
Due to butterflies to do with slavery, I was also planning to avoid the Great Trek in the 30s and have a Boer dominated South Africa break free from Britain during the revolution. I also am planning to give the Ottomans a second wind so they could retain some African possessions. Maybe have Portuguese Africa pass to Brazil in a British-backed deal after the Miguelites win the Portuguese civil war and the Pedristos marry back into the Brazilian Imperial family. Haven't researched anything on it, but maybe Oman could hang on to something too.
The problem is, to have many of these will create so many butterflies in Africa that they preclude the others, especially considering there are PODs in the 1830s. But this is for a Darkest Hour mod, so I'm not gonna go all butterfly fundamentalist over it. But if there are any glaringly obvious preclusions, please anyone point them out so I can try to accommodate.