THE death of US Commerce Secretary Ron Brown in a plane crash in Croatia on April 3 has cast a dark shadow over Washington. Brown was in the Balkans in his self-assigned role as the nation's chief salesman, seeking profitable opportunities for American businesses abroad and making the case that economic power should be one of the key cornerstones of US foreign policy. His three-day mission was aimed at promoting US exports, investment, and business participation in the reconstruction of the former Yugoslavia.