By rights, the fourth album by this band of earnest retro-rockers should be their worst. While staying true to their roots in ’60s Merseybeat-inspired guitar crunch, the Smithereens decided it’s time for the kitchen-sink approach on Blow Up. There are collaborations with Julian Lennon and songwriter-for-hire Diane Warren, plenty of string sections and horns, and, in the words of Capitol Records, the band’s first ”overtly political” song. Many a rock band has been crushed by such misguided moves, but lo and behold, the mess works. Production touches like seedy lounge organs open up the band’s crunchy, somewhat limited sound, and the Warren song, ”Get a Hold of My Heart,” has a glorious string-drenched chorus that sounds more like vintage Petula Clark than the romance-novel weepers Warren cranks out for the likes of Heart. Fans needn’t worry: The band’s droopy-voiced singer, Pat DiNizio, still writes sad-sack songs with hooks that sound as if he had an old British Invasion-era LP at his side, and the group still pile-drives through them as if they want to cheer him up. But four-piece, guitar-based bands rarely push themselves as far as the Smithereens do on Blow Up, and they rarely pull off such adventurous pop with such snap and crackle. A-