The menu bar on an inactive display (displaying a background window) is dimmed - grayed, if that's what you mean by "light black".
To get rid of it, my experience so far has been that the app has to be in full screen mode (as in an OS X full screen mode). Then, videos also played full screen within the app should fill the display (assuming a 16:10 display, you'll have black letter-box bars for 16:9 and wider aspect ratio video). Most of the time I'm in extended desktop mode, it's through AirPlay, but I believe the Display Preferences for the external display should be the same as wired.
If the display is a primarily a TV display, check the overscan settings in OS X. If on, you might be only getting part of the menu bar displayed. Some HD TVs also have their own settings for "overscan" to hide certain broadcast artifacts. This should be off, unless you actually see artifacts occurring regularly. Otherwise, the TV won't display the outer couple of pixel rows.
I'll have to try VLC on an external display. But, I've used the secondary display with Flash played through Safari (via LiveStream) just using the player's full screen mode and haven't noticed any artifacts. This is again through AirPlay (to an HD TV), though it seems to be implemented with the same Display Prefs as a wired display. If I get a chance, I'll have to try both players through a wired connection.
If you open an application on your machine and drag its window over to your secondary monitor, the translucent menu bar will *always* show on the secondary monitor. The external monitor is a separate space (unless you've deselected that option in System Preferences, which is kind of a silly thing to do), so if you drag a window over to the second screen, the application is actually running out of your machine's space and not from the secondary monitor's space. So in the eyes of the external monitor, the focus is still on your machine's space when you're running a dragged VLC or browser window (or whatever application) in fullscreen mode on the secondary monitor. So it displays that obnoxious translucent menu bar.
So long story short (too late!), literally all you need to do - there are other ways, but this is the easiest - is close the application, put the focus on your secondary monitor's space (by opening a finder window in that space or switching to an app already running that was opened in that space or using the dock from within that space, etc.), and then reopen the application from within the secondary monitor's space. Now when you go fullscreen, the menu bar won't be there, because the focus of the application is in the space it's running fullscreen in.
So in your case, if you make sure to open up VLC from within your secondary monitor's space and then go fullscreen, the black bar should disappear.