The revolving doors at Bill Blass haven't quite stopped spinningalthough Peter Som has been named as Michael Vollbracht's successor, his first collection will be for pre-Fall. The clothes shown today at the Celeste Bartos Forum in the New York Public Library were the work of the design team of Prabal Gurung, Ana Carolina Coelho, and Tyler Rose. For this interim effort, the trio dug into the house's archives and honed in on the party dresses they found there. Some of the results simply looked liked dusted-off versions of the originals, while otherslike a chiffon frock with embroidered scallops, a shantung gown with a sweetheart neckline, and a flounced-hem L.B.D.had a bit more appeal. The stage is set for Som's entrance.