Why is my older dog scooting, struggling to poop, and has a dark anus?

Poor Willow! Some dogs can experience pigment changes over time or with chronic inflammation. It's quite possible that the darkening of Willow's anus is a relatively normal change. As you've mentioned, scooting commonly occurs with full anal glands. I'm glad that you have a vet appointment scheduled for her. Your vet will be able to perform a rectal exam to be sure that everything feels normal, as well as empty her anal glands. It should be ok to wait 2 days since that's the next appointment. Full anal glands certainly aren't an emergency, but sometimes the anal glands can become impacted, infected or rupture, which would be a more urgent problem. If you notice any significant swelling or redness on her hind end, I would try to have her checked out sooner. I suppose that it's possible that she could have a perianal fistula, but these occur most commonly in intact male dogs and are often quite red and oozy. It's far more likely that her anal glands are simply bothering her. I hope that all goes well for Willow!