We have not seen a problem in the left leg in older swans and we have seen them as old as 26 years of age. The blisters could be the result of many things such as being burned while walking on heated surfaces, the beginning of an infection such as bumble foot or other infection, or even some nutritional issue. If the foot is sore, this might be the reason that you are seeing some weakness in the leg. She could have also sustained a soft tissue injury in the leg, foot or hip.

We would strongly urge you to get her to a veterinarian with swan or waterfowl experience as soon as possible to rule out any underlying condition
that could result in further illness or injury or a life threatening situation. The Regal Swan