What John Mayall did from 1964–66 probably couldn’t be attempted today: He launched a band of mostly-younger players, each of whom became a major force in English music over the next four decades. That batch of Bluesbreakers—with Eric Clapton, John McVie, Mick Fleetwood, Jack Bruce, Peter Green and Mick Taylor at various times—may be his most celebrated era, but it was only one of many dozen, wildly diverse blues bands he’...