Is it normal for a Chrysler 300C to have black differential fluid?

I just had my 2007 300C with AWD serviced and the guy mentioned that my differential fluid was black. My cabin air filter also needed to be replaced badly, which was done and apparently my oil pan is "sweating." Don't know if that's related When I fired it up again, I noticed some really hard shifts in the (NAG1 by Mercedes) which has never happened before, and then there was a really hard shift/clunk about 10 or 15 seconds after stopping at a stoplight (in idle) and the general downshifts on acceleration are really pronounced (like my old 01 V6 mustang...lol). The guy wanted to know if I did a lot of burnouts, which I never do and obviously is impossible on those cars with AWD anyway. None of these issues were related, apparently, to anything on the maintenance schedule. The car has 48,000 kms or 35,000 miles on it. I've never had a single problem with the car before. It's been a joy to drive and still under warranty until 2012. Any suggestions?