I have seen grease go black on the hubs with brakes. Changed to high temp grease and all good. Now use oil hubs and no problems.
I dont remember the stubs going black though! Some body will chime in soon. best of luck on your trip.
ps. once towed 3ton caravan from brisbane to appletree ck with the van handbrake on. Now that is hot.lining was wafer thin grease ok(?) stubs still good.