The plane does have N706BL painted on it very clearly and is visible both in the still picture at the beginning of the video as well as the actual takeoff. It is located right below the tail and between the veritcal stabilizer and the rear door.
It does have N706BL painted on it exactly where Northwest had painted their registration ID - on the rear of the fuselage. If you look close in the movie's second shot, you can see it - gray lettering over the white fuselage's tail end.