The maturation of the Black Keys as record makers and performers has been both subtle and startling. With their 2008 Nonesuch release 'Attack & Release' - the fifth album of their eight-year career which doubled the sales of their previous album and Nonesuch debut 'Magic Potion' - guitarist Dan Auerbach and drummer Patrick Carney illustrated the durability of their few-frills sound, a mysterious and heavy brew of seventies-vintage rock, classic R&B and timeless, downhearted blues. Producer and pal Danger Mouse, their first outside collaborator, didn't try to reinvent their sound but further isolated its essence with the help of a few carefully chosen guest players and some retro-modern electronic gear. It didn't need to get slicker to get better, or, as the Boston Globe put it, ''Attack & Release' proves that cleaning up the boys still won't stop them from tracking mud all over the house.'