Ms. Giddens’s visit — which included leading an afternoon workshop with inmates and performing an evening concert for them — was organized through Carnegie Hall’s Musical Connections program, which sponsors educational outreach at Sing Sing, helping participating inmates learn to play instruments and compose.  I just couldn’t be more grateful,” Mr. Blackwell said afterward, “that a man serving 20 years to life in prison gets this kind of opportunity, to just be human for a few hours.”