SANTA MONICA, Calif., Jan. 19, 2000 -- Will Della Reese's guardian angels also be looking over the shoulders of Blair Underwood? CBS execs and Steven Bochco, producer of the new series "City of Angels," certainly hope so. The drama, starring Underwood, Vivica A. Fox and a primarily black cast, focuses on the team of medical workers who save lives, among other things, in an inner-city hospital. Ever since its inception, the drama has been offered up as the network's response to public outcry (among them the National Association for the Advancement of Colored People) over the underrepresentation of racial minorities on network television. If the drama's debut ranking is any indication of the future success of "City of Angels," Underwood and company should have nothing to worry about. The medical drama, which premiered Sunday, took the second-highest ranking in its t