The oil in a diff CAN go black. If it has been punished it will become burnt, the more burnt it becomes, the more black it becomes.
As for Castrol oil - I would not touch the crap. I put Castrol oil in three seperate Skyline Diffs and all three developed a whining noise. One of those was my car and the whine got worse and worse until I changed it - for the third time. Castrol don't want to know anything about it because it is an Import.
I only ever recommend REDLINE Diff Oil. You should use a 90 or 140 grade. Places like Autobarn can get it for you or if you are in Brisbane, Thompson Spare Parts stock it on the shelf.
If you have to use Castrol, ONLY USE THE SYNTRAX AS SUGGESTED ABOVE. The top two listed will wreck your diff.