As shown in the figure, in both the moving and idle states of the character，I tried adjusting the parameters for fabric rendering, but it didn’t completely solve the issue.
Hello there @sshiny!
From what I can see in the screenshot, and the background provided, this would require further clarification, as there are two sections in the ribbons where a black color is present.
If you refer to this black spot at the end of the back ribbon, that look like the section is missing a side. My first guess would be to ensure your material has “Two Sided” enabled, or add a TwoSidedSign node to it. I would also send the mesh to Blender, and make a geo check (look for any non-manifold, recalculate all normals, etc).
On the other hand, there are a few dark spots in the side ribbons, these look more like a UV misplacemente, part of the texture is taking areas with a different tone in your map. For that, please check your UV seams and ensure that section of cloth is not overlapping with anything.
Still, if you can send more data related to your fabric render, we can investigate this further.