This question already has an answer here:
I've managed to render PNGs which have my machine on a transparent background, but I want to have a video of it. I've tried lots of settings to no avail....the film/transparent setting doesn't seem to do anything.
Every vid I've rendered, whether it be straight from Blender Render or Cycles Render or via creating a film strip from the PNGs via Video Editor and creating a vid from there, turns up with a black background from my scene.
Whats the trick?