Watching NBC’s Saturday Night Live in past seasons, it was possible to dismiss Dennis Miller as a smug twit, his smirk exceeded only by the length of his shaggy hair. This season, however, Miller has changed; it’s not just that his hair is shorter — he’s given his comedy a make-over as well. The political jokes in his ”Weekend Update” have become bitter and blunt, and in his new HBO special, Dennis Miller: Black & White, Miller goes even further-from bluntness to a dazzling pop-cultural complexity. The hour offers Miller doing his latest stand-up act to a nightclub audience. But he’s working on a level the nightclub crowd hasn’t seen since Mort Sahl in his prime — this is the most recondite comedy act around. It’s impossible to pin Miller down politically — he snaps at George Bush and animal-rights activists, and comes out in favor of the death penalty and against born-again Christian ministers (”I can see through these guys like used Neutrogena”). Jokes are made about Herman Melville, Bachman-Turner Overdrive, Marion Barry. Near the end, he yells, ”Stop me before I cross- reference again!” Not on your life, Dennis — more, more.