To watch a show by singer-dancer Daniela Mercury, one of Brazil’s biggest stars for nearly 30 years, is to plunge into a pulsing, hyperenergized fantasia of her hometown, Salvador da Bahia, arguably the most African city outside Africa. The stage teems with dancers in billowing Afro-Brazilian garb; a battery of drummers pounds out the rhythms of axé, the densely percussive pop native to Salvador that Mercury made famous.