It depends on the region. If you are talking about a tiny Masai village in the middle of nowhere than it’s fair to assume that the guy is clean and living off the basics.
If you are looking at African gyms in the major cities (Mombasa, Lagos etc) than I'm sure that they have access to steroids and other substances.
You will be able to find such stuff in every city in the world. I remember when Afghanistan held its first BB competition after the fall of the Taliban; the competitors were clearly long term juicers.
I know this guy in the gym originally from Zimbabwe and we were taking about Michael Clarke Duncan after his death. This guy was like "Yeah, he had health problems because he was taking steroids. Back home, lot of young guys take steroids to look like the wrestlers they see on TV and end up having heart problems"
Having said that, West African clearly have superior genetics as far as bodybuilding goes.