Dust, string, hair, etc in front of the film in the camera at the time of exposure leaves a dark line or mark, its "shadow" on the film during exposure where development can build no density.
Dust, string, hair, etc in the light path in the enlarger leaves a white line where it blocks transmitted enlarger light from hitting the paper, like the white triangular line near the top in the full frame print.
Scratches in the emulsion are normally sharper and straighter than the curved black line. The dark curved line in your print looks like the out of focus shadow of something inside the camera when the film was exposed.
I once had a very angry customer come back from an overseas vacation where he used his brand new Canon AE-1 for the very first time. Every frame he shot had a thin black vertical line, a tiny bit wider near the edge of the frame. I told him what I thought it was, a hair across the shutter, but I was wrong. There was a very thin thread of enamel paint, the interior finish on the camera, across the lens side of the shutter gate. A simple manufacturing flaw that I fixed in five seconds. He didn't run a test roll before his vacation, and didn't process anything until he got back.
You have the same kind of thing going on, but further in front of the film.
Lee