Canadian actress Rebecca Northan sat forlornly at a table on the stage of the Galleria Theatre wearing a tight dress and a red clown nose. She told us that her name was Mimi the Clown, that she had been stood up, and that she was going to select a new date from the audience. She did. And this wasn't one of those token audience-involvement gestures you see so frequently. This was the real thing. The man she selected at each performance spent almost two hours on stage with Northan, improvising his way through the getting-to-know-you chat, a snuggle on her sofa and an imagined five-years-down the line sequence — all under her shrewd, sometimes gentle, sometimes slightly sterner, guidance. A show like Blind Date, brought here by Denver Center Attractions, is a high-wire act, calling for instant judgment — everything depends on the man who's selected — and a knowledge of when to control and when to let the untried partner take the reins. But Northan's presence, charm and intelligence never faltered.