If real MCs are only found underground, then Mos Def and Talib Kweli, a.k.a. Black Star, are representing the real. The Brooklyn duo's debut is not only an eagerly awaited rap album but also a desperately needed one.

For Black Star, hip-hop is, simply, skills, self-awareness and love of their art – not Rolexes, Benzes and the Benjamins. Corny? Maybe. But Black Star would rather be corny than...