Jazz singers with celebrity cachet are one thing. Celebrities who purport to be jazz singers are quite another. You know the type: movie, TV and pop stars who share the Cybill Shepherd delusion that all it takes to carry the torch is a passable voice and a piano. Prominent among them, and a shade or two better than most, is Steve Blackwood. A familiar face to fans of daytime drama-he plays dashing Bart Biderbeck on Days of Our Lives-Blackwood has a pleasant voice that's eerily reminiscent of Peter Allen, and a penchant for Vegas-style showmanship. Listening to his latest album, Mood Swings (PosiTone PR 8008; 34:38), it's easy to envision him doing decent business in Sin City's smaller lounges. He handles chestnuts like "Honeysuckle Rose," "It's All Right With Me" and "Lady Be Good" with limited ingenuity but buckets of confidence. Included in the mix are two of Blackwood's own compositions. The first, "Lauren," suggests he's been spending too much time listening to the Broadway songbooks of Adler, Ross and Loesser. The second, "In the Heat of the Night," shows a lot more promise. It ain't Cole Porter, but it ain't half bad.