Here's the truth. Power steering fluid is good for well over 100,000 miles in normal usage. It changes color because it leaches carbon black and other chemicals from the rubber power steering hoses and fittings. Just because it is dark does not mean it is damaged.
When should you flush your power steering fluid? If it gets overheated (not sure how, perhaps failing pump?), or contaminated. And you don't need to power flush it, you can simply suck it out with a turkey baster and refill the reservoir. According to an expert on this subject, who I happen to know, there has been no proven benefit from power steering pressure flushing.
When the dealer pushes you to change your power steering fluid, he is really just trying to improve his profit margin on your sale.