Hello, my name is Mark and I will try help you with this.
If the fluid is black then it is a sign that it is contaminated.
The contamination could have been by someone accidently topping up with the wrong oil or by excessive wear somewhere in the system.
Either way, it probably means you will have to replace the entire steering system as flushing it out will probably not be enough and most likely secondary damage has occurred.
You may need to replace the steering rack and the pump and rinse out the pipework depending on how bad the contamination is and if it has caused damage to seals in the system.