Some voices are born to sing the blues, and Shannon McNally has one of them. It’s rich and rangy, colored with heartaches and tragedies. On Black Irish, she uses it to tease out emotional nuances, whether she’s murmuring through the bluesy stomp of “I Went to the Well” or invoking a fanciful drawl on a strutting Stevie Wonder cover. For all that formidable power, she can also pull off feats of exquisite fragility, as in her stunning, spare take on “Isn’t That Love?”