It's possible that the oil had some type of graphite added to it, or maybe some other additive that made it look like that. I don't know what is in the Auburn oil additive. That might be what it is. I'd let it go for now and see how it holds up. If it still makes the noise then drain it and start over with new oil and 2 bottles of GM or Ford additive.
If you ever decide to do this again, Eaton makes a very good Posi for that rear end. I highly recommend it over the Auburn.