Good to hear that your grandmother was seen by a professional before it was too late. Such complications can happen when there is an obstruction in the arterial system of the involved limb. Whenever there is trauma there will be associated hematoma
(collection of blood) in the tissues which starts to become dark during resolution but this should not be confused with gangrenous changes in the leg. I would suggest she meets a vascular surgeon who will be able to help you further.