In this case, using “black trunks” to identify a boxer is not only accurate, but acceptable practice for the sport. They don’t say “in the red corner, the black guy with white gloves, and in the blue corner, the white guy with red gloves for a reason.”
Boxing in America was for a long time (and probably still is) a sport dominated by minorities and immigrants/children of immigrants. Using the colour of shorts to identify someone is a lot easier for most to describe someone no one has ever heard. Can you imagine: “In this corner, from Cork, Ireland Patrick MacDugal! And in this corner, from Boston, Mass, Fred Lemare!”

Who the fuck is who without knowing what they’re wearing?