MARKET PLACE A drug aimed at treating heart failure specifically in black patients has proved so effective that the clinical trial in which it was being tested has been halted early, the company sponsoring the trial said yesterday. The results are expected to lead to approval of the first drug specifically for a single ethnic group. They seem to validate the gamble by the company, NitroMed, to take a drug that had failed to win approval for general use and test it only on African-Americans, an approach that ignited controversy on the relevance of race to medicine. The company's stock...