MINNEAPOLIS, MNAdvertising executives say they have hit upon an ingenious new way to target blacks: Mount a campaign that co-opts their own language and musical style. 'Many blacks enjoy 'rapping' music,' said Briggs & Adams Advertising president Sherman Roe, who developed the campaign for HospCare HMO. 'And what better way to tap into their market than by 'rapping' good health to them?' Roe's campaign employs the use of a black teenager doing a 'rap' for good health. Billboards of the campaign have been put up in black neighborhoods, and radio and TV ads have aired on black-oriented stations in the area. Roe predicts area blacks will, as a result, be ''rapping' happy with their HMO service.' 