It is not normal for the leg to turn purple when it is hung down early or late in the course of recovery. This suggests that there is something that is blocking venous drainage from the limb when it is placed in the particular position and may need evaluation for appropriate management. The hard spot could be a displaced bone, hardware or hypertrophied scar, which if symptomatic may need adjustment. It would be best to get these evaluated by your treating doctor for appropriate management.
Hope this is helpful.