I have a 250ci that is light on one side. When I run the color belt the black is light on the side. The black drum and dev have been changed. Any ideas?

check the slit glass on the laser. If you have light film on the laser it will cause this. They are not easy to get to unless you pull the laser.

Have you tried swapping out the transfer belt?

I did not try the belt yet because the yellow looks fine

Before you do anything, pull the the K drum and dev and really clean the slit glass, had a tech try to clean one with a greasy rag, and the back half of the prints were weak and a tad blurry. The blurriness gave it away for me, wiped with glass cleaner, prob solved. A laser with a fogged lead edge on the mirror will streak down the page about two inches in from the front. The lead edge will fog first, ( as it's lead edge hitting the crap first).
If it's not the laser glass, I would go for the xfer belt assy next. the K xfer roller under the belt may be dirty, defective.

laser is a good bet. had a 500ci laser go out, it had ozone on the mirrors inside the laser. this machine only had about 100k on it but was in a dusty location and the filters were clogged. take the drums out and shine a light though the slit glass and look for haze on the mirrors inside the laser. make sure to clean the filters,5his may stop this on other machines

It was a hazy film on the polygon mirror