There’s a cool charm to Girl of the Century (Bloodshot), the new collaboration between Austin “rockabilly filly” Rosie Flores and the Chicago-based Pine Valley Cosmonauts , that defies easy explanation. The Cosmonauts are a loose collective led by Welsh rocker Jon Langford (the Mekons, Waco Brothers), who is hardly a veteran of the Texas dance hall circuit Flores calls home. It turns out that’s a good thing. While there’s a helping of the hyperfueled rockabilly that is Flores’s meat and potatoes (Johnny Cash’s “Get Rhythm,” Bobby Darin’s “This Little Girl’s Gone Rockin’ ”), it’s the Langford-influenced selection of mid-tempos and ballads that showcases her versatility. The arrangements are nothing special, but on blues shuffles like “Chauffeur,” Langford’s “Halfway Home,” and Paul Burch’s “Little Bells,” Flores nestles into the swing rhythms and takes control. Better still, on ballads like “Last Song” and “Dark Enough at Midnight,” she dazzles without histrionics, drawling her way to notes like a countrified Billie Holiday. Such restrained confidence is rare in any singer, yet Flores makes it seem effortless.