I'm a blueser and open 'D' tuning is traditional in the blues. It was call 'Sebastopol' or 'Vestopol' from a ragtime piece popular when guitars became more common arund 1900 and began to supplant the banjo. Open 'D' is used by many of the greats of the blues (along with "G" tuning ["spanish"]. It has the same string/interval relationship as open 'E' tuning only with less string tension. Robert Johnson, Muddy Waters, and Elmore James all used this tuning, as well as others. And the great Blind Willie Johnson who I recommend even to the Celtic players.