A 64 year old man has swelling in the feet and leg with a sore that wont go away also his foot is starting to turn black. What are possible diagnoises?

Seek medical care. The swelling could be a venous issue, however with the discoloration you need to worry about poor arterial circulation and possibly impending gangrene. This gentleman should seek medical care promptly.
PAD. Severe arterial insufficiency. Wounds need blood to heal, non healing sores are a sign of inadequate circulation.
Sounds emergent. Any limb that is swells up and turns dark has likely lost part of all of it's blood circulation. That means an ambulance trip to the emergency room where the limb can be evaluated. Rapid treatment may be needed to try to save the limb.