The little preview picture in question is called the "poster frame", my guess is that the videos that are all black, just happen to have picked a frame that is all black, because the video fades in slowly or something like that. You should be able to set the poster frame to any individual frame of the video when you play it back, although this may require QuickTime Pro.
The little preview picture in question is called the "poster frame", my guess is that the videos that are all black, just happen to have picked a frame that is all black, because the video fades in slowly or something like that. You should be able to set the poster frame to any individual frame of the video when you play it back, although this may require QuickTime Pro.
No need for Quicktime Pro, just play the movie in a separate window in iTunes - you can use the forward button to progress slowly, when you get to a frame you like, ctrl-click and choose "set poster frame."
Some videos won't allow me to set the poster frame, but most will. All the videos were downloaded from the Internet and imported into iTunes (not purchased from the iTMS), and all of them are Quicktime .MOV files.