For the Brooklyn-based opera company which found itself in a hurricane of bad publicity last week after announcing that it would use a computerized orchestra for an upcoming production of Mozart’s Magic Flute, things just keep getting worse. At least one singer has quit the production for fear of being blacklisted in the opera world, and an e-mail campaign by the American Federation of Musicians is causing untold headaches. But the opera’s director insists that he would hire a real orchestra if he had the budget, and can’t understand why the musicians’ union would stand in the way of the development of young opera singers.