2001 Mazda Tribute power steering



I had my 2001 Mazda Tribute (4 cylinder) serviced at one of those quick lube & oil change places 1000 miles ago, at which time they found my power steering fluid to be black and sludgy. They said that they had to flush it with new fluid 3 times to get it back to normal. I don’t think that that had ever been done to the car at the dealership (I bought the car new and followed the recommended service to the letter). Now, 1000 miles later, I checked the fluid because I heard a very faint whirring or whizzing sound inside the car while sitting still and turning the wheel - it could just be my steering wheel - I don’t know. I cannot hear anything abnormal when I’m outside the car and someone else turns the wheel. The fluid is black and sludgy - and after only 1000 miles of very light use. What is going on? Is there a filter that needs to be changed or am I about to lose my power steering pump? Any insight anyone out there can provide will be greatly appreciated.