Any free way to screen shot scenes from a movie played on a mac?

In VLC, did you use: Menu Bar -> Video -> Snapshot?

And if you tried using the standard screenshot shortcut, use shift-command-4 then pressing space, then clicking to take a 'object' screenshot.

In any case, any of the ways works for me. Maybe since you have dual screens, OS X has two image files, one for each screen. One image is black because it's in fullscreen mode, and the other has the video?

Running an anime review site, I've taken hundreds of screenshots of both DVDs and video files. I usually use VLC, and have never had a problem with it on any of three different computers. If you're getting blank screenshots out of VLC, you've either got it set to some odd display mode, or there's something wrong with your install--try resetting the preferences to get it back to the default state.

MPlayer OSX Extended also works well if you're screenshotting a video file rather than a professional DVD, but for whatever reason the PNGs it produces aren't as well compressed, so they're larger on disk until you run them through a processor. Both look identical.

You are taking screenshots from within the app, not via the OS-level screenshot command, right? Command-option-S in VLC, Command-shift-S in MPlayer OSX Extended. You can just drag and drop a screenshot with QT Player 7, but it tends to save them as movie clippings that need to be further converted, so it's easier to use one of the 3rd party players.

[Edit: Ninja'd.]