Marquise Knox, the young St. Louis-based blues singer/guitarist who appears at the Attucks Theatre on Saturday, has been saddled with the label "manchild" because of his youthful look and young start in the business. But his version of the blues is anything but childish. "It's raw, none of this pop-blues," he said in a phone interview from his snowbound home. Now at the ripe old age of 20 Knox said he's moving away from his manchild image. "When some people first see me they say 'what'chu know about the blues?' I tell them I know exactly what you know, and I know how to apply the blues," he said. "They now consider me one of them. They don't look at me as a child anymore." The Granada, Miss., native grew up in a blues-singing family. The sounds of Muddy Waters, Lightnin' Hopkins, B.B. and Albert King, and John Lee Hooker filled the house. "I learned from my grandma Lillie Mae, my great-uncle Clifford and my great-uncle Joe, who bought me my first guitar," he said. "They told me when I was young, the blues was all around me. And when I was big enough, about 11 or 12, my grandma began to teach me the guitar. She knew the blues; her family were sharecroppers, and her great-great-grandparents were slaves. And I stayed with my Uncle Clifford for four years, and we played the blues every night." Soon Knox came under the mentorship of St. Louis blues legend Henry "Mule" Townsend, a Grammy winner and National Endowment for the Arts Heritage Fellowship recipient. "Me and Henry were go