Boxing hasn't been dominated by blacks since the 90s, plenty of whites, Slavic and not (canelo (representing Mexico but he is ginger), Tyson fury) are dominating even though there have been a few black fighters of note, blacks also choose to go into athletic careers much more often as its one of the only things they excel at, most other races are dissuaded from athletic pursuits, wrestling isn't, and there is many good white fighters in mma, the arguably best ones such as GSP, Khabib and Stipe (for heavyweight), mma has been dominated for whites for most of it's existence, Jon Jones is an exception to the norm and has been busted doing drugs multiple times. Blacks were in fact pushed out by Eastern Bloc whites in boxing during the 21st century.

Whites are often slightly stronger in the upper body due to proportions and muscle composition and have greater endurance (than west African blacks) due to muscle composition and lung capacity, blacks have more fast twitch and lankier proportions to be fast. Whites are also typically stronger pound for pound, as they have slightly less muscle but statistically have at least the same muscular strength as blacks.

Whites also have more even fat distribution and more fat than blacks, and longer torsos, bone density is often cited as the cause for black underpreformance in swimming but neither race typically has more or less dense bones in males overall, most studies have a limited range of radiography only measuring the lumbar spine or hip and who has denser bones may depend on which part of the skeleton you measure, such as whites having denser bones in the rib cage but less in some parts of the spine, also bone density isn't really much of a factor.

Blacks do have advantages in some sports, such as ones that require running and jumping, although American blacks typically outpreform African blacks in most sports except ones such as distance running, there has been little evidence of slaves being bred to be bigger and stronger, they would surely dominate wrestling and strength sports then?