Thanks for the query.
increase in melanin
pigmentation is common.
But usually these will decrease with passage of time.
As your black spot is changing the color and causing pain etc. better to get examined to find out the actual nature of the spot.
That may not be due to kidney
If you once consult dermatologist
and get examined.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429