They’ve certainly earned their reputation as the world’s greatest rock ‘n’ roll band, but the Rolling Stones have always been, at heart, a terrific blues outfit. It’s the genre on which their empire was built and the bedrock of their swaggering identity. It’s their first, most abiding love. And now, over 50 years since they formed, Mick Jagger and co. have returned to mother on a youthful and exuberant album of blues covers for the connoisseur. That’s right, Stones purists: Christmas has come early.