The fluid will also get dark with time, so dark fluid alone means nothing. Note that Mercedes-Benz has specified NO maintenance procedure / mileage / time frame for changing the fluid. There are many wagons out there that have never or very rarely have the fluid changed, and that fluid is black.

If you change the fluid to new, and within a month or three it has turned dark, then you have a problem.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC