I drained my front diff oil at about 945 miles and found it to be black and there was about 1/4-3/8" ball of black sludge on the drain plug. The oil was always clear when I drained my 800. What would have made the oil black? Has anyone else see this?

I drained my front diff oil at about 945 miles and found it to be black and there was about 1/4-3/8" ball of black sludge on the drain plug. The oil was always clear when I drained my 800. What would have made the oil black? Has anyone else see this?