Here is another simple effect in Darktable: Dark Grunge Portrait.
This tutorial is reproduction of nice video on GIMP Dark-Grunge-Portrait-Retouching – Gimp 2.9
The photo for retouching was used the same as in video.
Tasha https://www.flickr.com/photos/robnas/5439810712 by Robert Bejil available under a Creative Commons licence https://creativecommons.org/licenses/by/2.0/
In Darktable we need to make three simple steps:
1) Tone curve for shifting black point and make image dark and contrast.
2) Channel mixer to convert image in B/W and blend it in multiply mode (use "B/W reduce blue artifacts" as base)
3) Simple retouch with Equalizer to smooth skin and increase fine detailed (sharpening)
Darktable style is here Dark-Grange effect.dtstyle (1.3 KB)