Well, people speak about the fast-twitch muscle fiber thing, but let me tell you, they make too many assumptions. The only study I've been able to find biopsied the THIGH muscles of black and white college students and examined the results. No word on whether or not the two groups had equal amounts of athletic training, etc. Anyway, this study found that on average, 57% of the White kids and 70% of the Black kids muscle was Type IIA (The so called "fast twitch" fibers...actually, there are two kinds of "Fast Twitch Fibers", IIA which contracts somewhat faster, and IIB which contracts slower but more strongly and with greater endurance. True Slow twitch fibers are the kind you get from say, Marathon running.). That's not a huge difference. As it relates to boxing, we must also consider that the differences in the upper-body are probably not as great. Black athletic advantage in sprinting after all, comes from lots of little advantages coming together, not just one type of muscle fiber. The fastest hands in boxing right now probably belong to Joe Calzaghe. Old timers who saw both fight used to say that Ali had the quickest HW hands "Since Dempsey". And the Klitsckos are lighting fast for being huge guys. So theres lots of exceptions to any rule you want to come up with.
It's also not true that Blacks have quicker reflexes than Whites, actual studies have shown the opposite, and Whites do dominate most sports where pure reflexive speed is the test (driving, fencing), think of who most of your ace pilots are.