First check the export format. Ordinary H.264 MP4 does not carry the reusable alpha channel expected for this workflow. Use an alpha-capable WebM, ProRes 4444 or HEVC-alpha output. Not sure whether a file is transparent? Open it in the free transparency checker: if it shows a checkerboard behind the subject, the file is fine and the player is the problem.
Two kinds of black background
Black pixels are baked into the file
The export is opaque. Moving it over another layer will keep the black rectangle because there is no opacity data to reveal the layer underneath.
The player displays alpha incorrectly
The file may contain alpha, but the preview application or browser does not composite it. Test the same file in a known-compatible destination before reprocessing the source.
A quick diagnosis
- Check the container and codec, not only the filename.
- Import the file above a colored layer in an editor that supports alpha.
- If the color shows through, the file is transparent and the original player is the limitation.
- If black remains in the editor, export again with an alpha-capable codec.
Choose the replacement format
Use WebM with VP9 alpha for Chrome, Firefox and OBS. Use ProRes 4444 MOV for professional post-production. Use HEVC with alpha when targeting compatible Apple playback.
What the checkerboard means
A checkerboard is a preview convention, not content that should be exported. A trustworthy background-removal product must show the checkerboard in the interface while writing zero-opacity pixels into the output file.