My gearbox fluid was as black as diesel oil when I changed it, and that was done at 168,000 miles.
You have good advice above, before you spend any money, follow it. The gearbox electrical connector is a good place to start, when those seals fail the ATF will wick its way up the cable and into the ECU, making a right mess of things (although it can be cleaned and restored 100% with alcohol or lighter fluid).
Oh, and changing the ATF would be a very good idea. Get yourself a dipstick, and read this:
http://forums.mercedesclub.org.uk/showthread.php?t=25566