Kurtis Blow is, somehow, only 67 years old. To put that in context, Jay-Z, whose debut album came out in 1996, is about to turn 57; Run, who introduced himself to the rap world as “the Son of Kurtis Blow,” is 61. But the Harlem native started early. At the tail end of the 1970s, he became the first rapper signed to a major label; his debut single, “Christmas Rappin’,” was a narrative counterpoint to the ubiquitous and comparatively freeform “Rapper’s Delight.” Overtly comical though it was, the man born Kurtis Walker was pulling from a rich history. His communications classes at CCNY had focused on the West African griots, who had travelled from village to village and reimagined the narrative power of song.