If the mask was feathered, those repairs would be less obvious.
Video inpainting algorithms are temporally unstable and somewhat unpredictable. Potential artefacts would be more distracting than black corners IME. You might be lucky. If there's lots of movement, motion compensation to fill in the missing bits might work better.
Where it's darker, but not black, a masked graduated luma change would help. That would leave a smaller area to repair (what ever method you choose for the repair).
Cheers,
David.
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