Fashion is not only invading television and museums; it’s on the New York stage, too, in the form of Paul Rudnick’s play, “Regrets Only,” whose central character is based on the late designer, Bill Blass. Rudnick says he’s surprised by the public’s fashion savvy — but should Rudnick have been a little more savvy about who Blass was? Cathy Horyn, who edited Blass’s memoir, seems to think so.