Let's dispel a couple of myths here:
Diet - can't be a factor because, in the world of high level athletics, diets are all uniquely tailored to the athlete's needs. Is it really plausible that black athletes know about a secret sprinting diet that they're collectively hiding from white athletes? No. Furthermore, Black athletes from many different countries have been successful sprinters.
Slavery - there are 8 Nigerian athletes who have broken the 10 second barrier, and others from Ghana, Zimbabwe and Namibia (compared to one white athletes). If sprinting ability was down to 'selective breeding' of slaves, then it wouldn't be present in West African countries - it would only occur in the descendants of slaves. The genetic advantage appears to be possessed by those of West African descent, and is therefore present in both West African nations and in black populations descended from slaves.
Geography question: Is West Africa particularly elevated? I was under the impression it wasn't, and that those people talking about altitude are actually confusing West African sprinters with East African distance runners. As sprinting is primarily anaerobic, the benefits to aerobic capacity that result from living and training at altitude surely shouldn't have that much of an effect?