A black background does not prove that a video is transparent. It may be ordinary black pixels, or it may be a player's backdrop behind an alpha channel. Verify the file's transparency support and composite it over another image in the application that will actually use it.
A green-screen video is a third case: it contains a colored background that still needs keying. Renaming a file or changing its thumbnail does not create alpha.
Agree on the receiving workflow first
Ask which editor, presentation tool or playback system will receive the overlay. Identify the supported container and codec, and whether that application handles the chosen alpha format. A file extension alone does not settle these questions.
For one concrete example, Apple's ProRes documentation identifies ProRes 4444 and 4444 XQ as the ProRes variants that support alpha. Choosing ProRes 422 instead is not an equivalent transparency export.
This does not mean every recipient can play a ProRes overlay or that every MP4-based workflow is necessarily opaque. Match the specific export to the receiving software rather than making a universal claim from the extension.
Distinguish pixels from opacity
Import the original export into a suitable compositing editor. Put it above a bright solid color, then above a dark color or patterned image. The background layer should show through the intended transparent region.
If a black rectangle remains, check whether the file has alpha, whether alpha was included at export and whether the application is ignoring it. If a green rectangle remains, complete the keying workflow before treating it as a ready-to-use overlay.
A browser preview or a stand-alone player can display a black backdrop even when the file has useful transparency. Conversely, a checkerboard painted into the video is still visible content. Judge the composite, not the backdrop alone.
Review edges during movement
Play through hair, veils, smoke, fine lettering and motion blur. These can contain partial transparency. Pause at difficult moments, but also review normal-speed motion for flickering edges.
Adobe's footage-interpretation guidance explains straight and premultiplied alpha. A mismatch can produce colored or dark halos at semitransparent edges. Check the documented interpretation in the external receiving editor before trying to erase every fringe.
Avoid removing all dark pixels indiscriminately. That can also remove genuinely black clothing or lettering in the subject.
A fictional wedding-overlay handoff
A designer plans an animated leaf ornament for a wedding film. The preview has a black background, so the editor cannot tell whether it is ready for compositing.
The planned acceptance test imports the delivered file above a cream card and then a dark blue card. Leaves must remain intact while the surrounding region reveals each card. The review also watches the moving tips for dark fringes and checks that the clip's duration matches the agreed insertion.
This is a fictional test plan, not an exported or verified overlay. If the receiver cannot decode the chosen format, the parties agree on another supported delivery rather than sending the same file with a different name.
Keep an overlay master and a viewing copy separate
A convenient opaque preview can help someone approve the animation, but label it as a preview. Preserve the actual overlay master and a short note about its codec, alpha interpretation and intended receiving application.
Plan the animated scene in Imagild Film Studio. Complete keying, compositing and transparency export in a compatible external workflow, then inspect the file the recipient will use.