Pat Benatar has just released the comedy record of the year. On it, she sings the blues perhaps the most uproariously dumb idea for an album since Ethel Merman recorded disco in 1979. Then again, Merman was kidding. Benatar intends no such levity. In interviews, the 38-year-old singer indicates with a straight face that this peerless bomb, in which she takes on brassy tunes by the likes of B.B. King, is her ticket to grown-up respectability. Of course, given Benatar's disappointing performance on the charts over the past half decade, the woman has cause to sing the blues. But unfortunately she doesn't have anything like the chops to do it. She's out to become the ultimate tough blues mama, someone who's been singing for decades about hard liquor and harder men. All she does well, though, is sing loudly. Admittedly that was the main requirement for her former persona as she-wolf of suburban hard rock, but here when she sings something like ''I Get Evil'' you get the idea that the worst sin she could imagine would be purposely not to tell her friends about a sale at Neiman-Marcus. At times her backup band (the respected Roomful of Blues) really burns, but Benatar still makes everything sound like the funkier parts of Suzanne Somers' Vegas act. Still, we really should be grateful for an album like this. We'll be talking about True Love for years.