On this inspired swing through soul, blues, / and jazz classics such as Al Green's ''Let's Stay Together,'' B.B. King's ''The Thrill Is Gone,'' and Billy Eckstine's 1945 hit, ''Cottage For Sale'' Flack sounds as supple, dramatic, and warm as ever. Sure, there are few bows to contemporary R&B in terms of form or content, but that's okay: After all, a singer of her standing is above kowtowing to trends. A-

