It's not just supply, it's support roles overall.
During Desert Storm the media was trying to push the old lie they pushed about Vietnam - that the Army was only sending blacks to fight the war. A CNN reporter was in Saudi Arabia with bleachers full of black soldiers trying to push the issue...then across the bottom of the screen it listed the unit he was interviewing as some sort of support battalion.
This prompted another network to examine the issue (it was either ABC, NBC, or CBS) and they concluded support units are full of minorities because they're all poor city folk with no futures or education so join the military looking for job skills, and white soldiers are the majority in combat arms because they're all rich entitled kids with nothing to do so join looking for adventure.
I served in Cavalry and Infantry battalions. HHT/HHC was majority black (support) and the line Troops/Companies were majority white. Hispanics were the 2nd largest group and in my opinion blacks were underrepresented in line units, so the above paragraph seems to have some truth to it. Either way, if true, good for them. Serving 4 years and getting out with no usable job skills sucked.