Rich black tones appear grey and noisy after export

Hello, everyone, I am pretty handy with a still photography, but new to videography. Shotcut is superb for my needs.

I shot a video of a black subject (a causassian with dark brown hair wearing black clothes) against a black background, 45 degree backlighting. The raw footage looks great, exposed correctly so that the black tones are nice and rich. However, when I edit the video in Shotcut (add a front bumper, rear bumper, separate sound track cleaned up in Audacity), and export the video, the rich black tones of the raw video become grey and busy with visual noise. I did not touch any of the settings on contrast, tone, or anything else.

I then tried to adjust these visual variables (lowered brightness, increased contrast, etc.), but each time the video comes out gray and noisy. Is there any export setting that is particularly suitable for rich black tones? Many thanks in advance.

@JanSabo What are the file specifications of your raw video? And what are the specifications of your final video?

Do you get the visual noise when playing back the exported video with Shotcut and/or VLC Media Player?