If the Buena Vista Social Club had consisted of just one member, the documentary about it might have looked a lot like I'll Sing for You. The man in question is Boubacar Traoré, who was known in the early '60s as Karkar, the Elvis of Mali. We see him in vintage clips and guitar-stud photos, singing anthems of independence, and then later, in 2001, when he returns to his native African land as an aging folksinger, with a trancelike voice that merges with his curlicued guitar playing almost as if it were another string on the instrument. Traoré is a hypnotic musician and, from what we can tell, a gentle and pensive soul, but the truth is that we learn very little about him. ''I'll Sing for You'' is one of those sanctifying docs that rambles when it should explore.