Denoise before or after color corrections?

Why not test it both ways? However, be aware if you apply this denoiser in FCP before you color, you'll need to bake that filter into every clip you apply it to.

With this in mind, I think I'd apply it after your work in Color.

Better yet would be to reshoot with proper lighting and mindful of protecting your shadows and highlights.

Gee, usually crushing the blacks is a cheap way to "de-noise" a picture.....


jPo

I guess is a physical thing

I think it's mathematical. 🙂 It sounds like you need to clamp the blacks to 0.
Try the FCP filter Effects > Color Correction > RGB Limit
and set the Minimum RGB Limiting to 0.

Color does clamping, too. The Color FX room has a clamp filter,
and the Primary Out room has a clipping function.

Clamping is to prevent the blacks from going below 0.
Sounds like your blacks aren't totally crushed. Try jumping up and down on them.
Seriously, you can get those last few dark grey pixels by using FCP's
Video Filters > Channel > Arithmetic, setting the menu to Floor,
the Channel to Color, expand the color disclosure triangle,
and move Brightness up a little. You can see it on the waveform monitor.

The reason I suggested clamping is that sometimes (in float color space),
when you push the blacks below zero, they wrap around and become visible.
I didn't realize your problem was that you had pixels above 0.