oil is black immediately after oil change

[rv4jesus]

I have a 2010 Prostar with a Cummins ISX motor and the oil is black immediately after the last two oil changes. At first I thought the shop didn't change the oil correctly but a different place changed it last time and it looks black as soon as you can pull the dipstick after letting the motor run for a minute. As far as I know twelve gallons were added and all oil filters were changed. I change my oil between 12,000 and 15,000 miles. What is causing this? Any help is appreciated.