On MacBook Pro (2.4/2.2GHz) computers, the icons for video clips in iMovie sometimes appear dark in the Clips pane when they are recorded with the built-in iSight camera.
The first time you use your built-in iSight camera, the camera may need to calibrate itself. As a result, the first few frames recorded by the camera may appear dark.
If you have footage recorded when using your camera for the first time, use iMovie to remove the first few frames that were used to create the icon, or try recording the footage again.