A free blood test for exposure to the acquired immune deficiency syndrome virus has stirred considerable interest in San Francisco. The city is prepared to screen 21,000 people for traces of the AIDS virus during the next few months at the test centers, which were designed to discourage people from going to blood banks, where plasma is routinely screened for AIDS. A positive test reveals antibodies that were stimulated by the AIDS virus, but it does not predict if a person will get AIDS. Therefore, some psychologists say, the test may cause severe emotional stress on those who receive a positive result. They say those who have been exposed will never know if the fatal disease will develop--until it does.