David Post and Craig Blackwell have an earnest, seemingly artless American folk sound: On the duo’s six-song cassette, their appropriately reedy voices harmonize to the accompaniment of guitar, piano, mandolin, banjo, and dobro, and Blackwell demonstrates a gift for timeless, ungimmicky songwriting. Yet close listening reveals that these D.C. folkies twist the genre in interesting, albeit gentle, ways. First of all, they call themselves not Post and Blackwell but rather Bad Dog. And while their rewrite of Dylan’s “Stuck Inside of Mobile” is radically traditional, “Baltimore” ends the program with decorous peals of feedback. It’s hard to imagine the duo getting trendier than that, but the tape’s stylistic assurance suggests that further experiments would likely be a success. The tape is $5 from Bad Dog, 3225 33rd Place NW, Washington, DC 20008.