Some camera's when taking videos, starting and stopping a clip, etc. will add a blank frame at the beginning of the clip. If you camera does that iPhoto uses the first frame for its thumbnail.
To check a video to see if that's the problem play it with Quicktime Player from outside of iPhoto and check the first frame. If it's black then you know the reason. If not then copy the video file, movie it to the desktop and try importing again.
If it still creates a black thumbnail try this: make a temporary, backup copy (if you don't already have a backup copy) of the library and try the following:
1 - delete the iPhoto preference file, com.apple.iPhoto.plist, that resides in your
User/Home()/Library/ Preferences folder.
2 - delete iPhoto's cache file, Cache.db, that is located in your
User/Home()/Library/Caches/com.apple.iPhoto folder.
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3 - launch iPhoto and try importing the video file again.
NOTE: If you're moved your library from its default location in your Home/Pictures folder you will have to point iPhoto to its new location when you next open iPhoto by holding down the Option key when launching iPhoto. You'll also have to reset the iPhoto's various preferences.
If deleting the preference file fixes the problem you can try rebuilding the library with the options to rebuild the thumbnail files, i.e. options #3 and #4.
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