That describes exactly how I found mine too. My truck is mint and well maintained. I do not use the auto-trac (I hard set the 4wd in or 2wd, never use 4-low) but my wife loves it. So she uses it maybe 3 times per year and she drives very carefully and not abusive. When my truck hit 80k (kms so 50k miles the factory OCI) and I dumped the fluid, was I ever shocked to find it black as well. There is nothing wrong with my transfer case and / or the auto-trac function but previous 4wd GM's I have owned with the strictly manual transfer case that uses regular Dextron, the ATF that has come out was like brand new. I wish I knew what the auto-trac elixir was exactly so I would have alternatives to use but until I figure that out, I use the OEM GM fluid. I get it on sale for around $5 or $6 / litre so it doesn't matter too much that I am bound to using it.