One of the more felicitous Chicago Symphony appointments of recent years is that of Michael Morgan as assistant conductor. Only 31, the Oberlin-trained maestro has made tremendous strides since joining the orchestra in '86. Poised, intelligent, and keenly astute, he is starting to fashion his own musical personality--that benchmark of true musicianship. Interpretively, he may lack the finesse, the wisdom, and the emotional depth of his elders, but as he's shown on several occasions of late, he knows how to shape a performance, making it at once exciting and coherent. The CSO obviously has high hopes for this up-and-coming conductor and seems to have found a perfect niche in which to nurture and showcase his considerable talent. In his capacity as goodwill ambassador Morgan will lead the CSO in a pair of concerts this weekend at Christ Universal Temple. Designed to highlight black participation in classical music, both programs feature prominent black composers and performers. The Friday evening fare includes the first composition by a black American performed by a leading orchestra: William Grant Still's evocative (and ambitious) Symphony no. 1 (Afro-American). Among the soloists are soprano Geraldine McMillian and baritone Ben Holt (paired on five songs by Richard Strauss). And the spirited 90-voice Christ Universal Temple Ensemble will be on hand for a number of spirituals and operatic favorites. Tonight, 8 PM, and Saturday, 3 PM, Christ Universal Temple, 11901 S. Ashland; 435-8122 or 568-2282.