Can't "see" any reason why?
You may want to include a picture, and also post this on the duck thread where the duck experts can help. The scrape may or may not be part of the problem, but bumblefoot, which are seen as redness, swelling, and a dark spot on the bottom of foot and toe pads, can spread into the joints causing infection. She may have just sprained her foot in a jump, or she could possibly have mycoplasma synovitis (MS) in her leg. I'm not familiar with ducks, bu t Amiga and others are. Here is some reading about bumblefoot, joint infections, and MS: