Fade movie to black, iMovie 9??

Can anyone help me please?

I want to fade the end of my movie to black but keep the sound from the clip playing. I have a 2 minute or so clip of video footage with sound, about half way through I want to fade out the video so all that is left is a black screen and the sound for the last minute.

Does anyone know how I can do this? The transitions don't seem to be any help.

Many thanks