I'm not sure about the blue. ..not seeing that on the chart. ..
Blue is who was selected for that branch without Branch Additional Service Obligation (BRADSO), i.e. with only the minumum commitment. So, look at Engineers (EN). The first to pick EN was the 9th ranked cadet. The 551st ranked cadet was the last cadet to be selected for EN without BRADSO. In the parlance of USMA, EN "went out" at 551 without BRADSO. Compare that to Medical Service (MS) which went out at 212, Aviation (AV) which went out at 234, and Military Intelligence (MI) which went out at 242, without BRADSO. No cadet needed BRADSO to "get" either Field Artillery (FA) or Chemical (CM).
Basically, you have to be ranked high in your class to obtain MS/AV/MI. FA/CM/TC not so much.
Any of the cadets with ranks (Order of Merit List - OML) in the red received their branch with an additional service obligation.
***That's last year's chart BTW. The 2018 chart was posted earlier in the thread.[/QUOTE
Thank you!