Good luck with that wrote:
"Yellow fever"
Your 2nd question isn't really relevant (but to answer: never - although Louis Hinchliffe is mixed Asian and white and did so this year, I seem to recall a fully Japanese 200m medalist from 2003 as well. And of course we have Su Bingtian). You're using confirmation bias, there are plenty of people with 2 black parents without the genetics required to be Michael Norman, and plenty of people with 2 Japanese parents with better sprinting genetics than several demographics of fully black people. Simply having a parent of a certain race guarantees nothing.
Less than a handful of non black people who are solid sprinters (no medal winners shown) vs, score (hundreds?) of black sprinters who have run faster.