Mention open tunings and some guitarists’ eyes glaze over – especially if they’re from the rock world, where standard tuning lives up to its name or, perhaps, dropped D or even-tempered half- or whole-step across-the-fretboard detuning comes into play. But in blues and folk music, and in some of Led Zeppelin’s greatest recordings like “Kashmir,” which is in D-A-D-G-A-D, open...