Your argument was not "as you go down Africa, HIV prevalence increases." Nobody would dispute that. Your argument was the cause of that phenomenon and is based on literally nothing but conjecture, which is problematic. This is particularly the case because the vast number of present day cases of HIV in southern Africa are from sex, at least according to the UN (http://whqlibdoc.who...1502986_eng.pdf).
The vast majority of people newly infected with HIV in sub-Saharan Africa acquire the virus during unprotected heterosexual intercourse (including paid sex) or as newborns and breastfed babies (via motherto-child transmission). Having unprotected sex with multiple partners and having other sexually transmitted infections (especially genital ulcers caused by herpes simplex virus type 2) are the greatest risk factors for HIV infection in this region.
Other factors described by the UN as major contributors: Unprotected paid sex, sex between men, injecting drug use, in descending order of importance in sub-Saharan Africa.
Contaminated blood in South Africa is not, and likely has never been, a significant issue in the spread of HIV (Note: It could have been an issue elsewhere in Africa, particularly early on, but South Africa got hit hard with HIV far enough after the risk of blood transfusions were known that most had blood testing in place by then). You are describing a conspiracy of quite large magnitude to create and maintain the epidemic, while covering all of that up for more than a decade, considering they would need to find a way to silence all of the people that would get HIV from blood transfusions.
There is no evidence to suggest such a conspiracy exists, merely a correlation between a white minority HIV prevalence. That is not sufficient evidence to blame a white minority for contaminating a blood supply.
FWIW, if you want to argue that the Apartheid government created socioeconomic factors in that region that greatly impoverished huge percentages of the population and made them high-risk for HIV, then you'd be more likely to have an argument to be made. I'm pretty sure I've previously linked papers that deal with the factors that contribute to HIV risk, many of which are driven by poverty.