Minority business owners have always struggled to secure bank loans. Now, many banks want to deal only with existing customers when making loans through the government's $349 billion aid package. Anticipating that minority business owners could struggle to tap federal aid, some lawmakers are proposing ways to earmark additional funds specifically for minority-owned businesses. And on Wednesday, a group of prominent black investors, including John W. Rogers Jr., the billionaire co-chief executive of Ariel Investments, a mutual fund manager, sent a letter to lawmakers expressing concern that the emergency loan program was already leaving black borrowers behind.