Just to make sure, I would feel and look over the leg above the dark area for any hair or string that might be wrapped around it. That should also cause swelling though, so there probably isn’t. Bruising from injury might be the answer, since she is limping. Let us know if you notice any...
What do the footpads look like? It is hard to tell if one foot or leg is more swollen because of the picture angle, but she might have sprained or fractured her foot or lower leg. The leg foot seems darker and swollen. Seeing the feet side by side would be a better picture if possible. If a vet...