For some 30 years, they embodied “country blues” for folk music audiences around the globe. Sonny Terry and Brownie McGhee were once ubiquitous, and as such tended to be taken for granted in the halcyon days of the 1960s blues rediscoveries. But nearly two decades have passed since the perennial team parted, and the 16 performances here remind us of this superb duo’s complementary strengths.