Why do old people's feet swell up and turn black?

Swelling with a : Dark discoloration is usually due to venous congestion... The pooling of blood in the feet and lower legs as a result of gravity and incompetent veins. A vascular surgeon would be a good starting point for you.

Turn black=venous: Old people's feet swell up b/c foot swelling is more common as one ages.The kind of foot swelling more often age-related is the chronic or recurrent foot swelling. That the feet 'turn black' when they swell, you probably mean they turn dark colored, w/c suggest venous stasis, a condition when blood pools in the feet due to valvular incompetence in the venous system (in the legs, ankles & feet).