This is likely a normal result after the trauma. That being said, you should be evaluated by your doctor
a few weeks after the event to ensure normal healing. I would recommend that you discuss this with your primary care physician.
Trauma can injure a number of structures in the leg. The x-ray (while not a perfect test) suggests there is no bone fracture. However there are many other structures like muscles and blood vessels that can be damaged. The fact that you have a black and blue leg suggests that there was damage to the small blood vessels -- this sounds like a bruise or as its medically known: an ecchymosis.
The small blood vessels break and cause the blood to leak into the skin and cause the ecchymosis. There is no way to fix the small vessels -- they must heal on their own. The blood must slowly be removed by the body -- but often before it does gravity makes the ecchymosis move toward the foot. This means the black and blue move lower and the swelling moves lower.
If you are having pain that persists, this may be a sign of muscle or ligament injury
. If this is the case see your doctor more urgently. If it is just the color that is bugging you, this is likely normal. Again, please discuss this with your primary care doctor