Dan Curtis’ gothic soap opera, Dark Shadows, began airing on ABC in 1966, with a live TV approach whose low production values and performance levels often were as frightening as the vampires and werewolves. But a supernatural soap? Curtis was on to something, and once the show started playing with different time lines and doppelgangers, you could really sink your teeth into it. So this weekend’s Decades marathon picks up the action, and the daily soap series, at M