It is no surprise that Willie Nelson has recorded a blues album; his is a career that recognizes no musical barriers. The question is, what took him so long? Nelson has always seasoned his country muse with a subtle pinch of blues, but on Milk Cow Blues the latter is the main course. Like everything else he plays, Nelson’s blues are unforced and natural. The well-chosen program features material associated with fellow Texans Bob Wills (through whom Nelson first learned Kokomo Arnold’s “Milk Cow Blues”), Stevie Ray Vaughan (who popularized Larry Davis’ “Texas Flood”) and Charles Brown (a shiveringly good version of the late singer’s “Black Night”). Given the record’s Lone Star-heavy flavor, it’s odd that there’s nothing here by T-Bone Walker, but the late guitar slinger’s sensibility is nonetheless evident in the mellow timbre of the single-note solos and jazz-blues chords played by Nelson and his guests. On board as duet partners and sidemen are legends (B.B. King, Dr. John) and youngbloods (Jonny Lang, Kenny Wayne Shepherd) alike. But the show belongs to Nelson, who has fashioned from these blues a sagebrush poet’s autumnal meditation on faded love and the wages of the sporting life. The result is emotionally rich, musically savory and languidly blue from end to end.