I haven't messed with video yet so I can't give you a full explaination of how to do it, but you should take a look at these commands since they are the native Blitz DirectShow commands.
OpenMovie
CloseMovie
DrawMovie
MovieWidth
MovieHeight
MoviePlaying
Found under Help/Command Ref/2D-Catagory/DirectShow
OpenMovie
CloseMovie
DrawMovie
MovieWidth
MovieHeight
MoviePlaying
Found under Help/Command Ref/2D-Catagory/DirectShow