Why does my black paint get swirl marks even when I hand wash carefully?

Yes, drying alone can absolutely put those marks in black paint, expecially the long dragging stroke you're describing. When a towel is loaded with water and you pull it across a panel with any real pressure, you're grinding whatever dust and grit survived the wash right across the clear coat. On a dark car that shows up as exactly what you're seeing, faint circular marks that only pop in direct sunlight. Your wash process sounds solid by the way, the two bucket routine isn't the problem. The fix is mostly about pressure and lubrication, not the towel itself. Either blot the water instead of dragging, or use the lightest touch you can manage and let the towel do the work with its own weight. Keep flipping to a clean side constantly, and swap to a fresh towel well before the first one gets saturated. I also mist the panel with a quick detailer or spray wax while it's still wet so the towel glides instead of grabbing. Dry microfiber on dry paint is where a lot of the damage happens, so never chase a spot with a dry towel. The blower question, yes, it genuinely helps. Getting 80 or 90 percent of the water off with air means your towel barely touches anything but damp paint, and it also dries out the mirrors, badges and panel gaps where water loves to drip back down and ruin your work. You don't need an expensive one either, a decent leaf blower on low does the same job. On soap, don't waste money on a fancy black paint shampoo. Any pH balanced wash soap with good lubricity is fine. Marketing fluff, mostly.

One honest caveat, even a perfect technique will leave some marks on black over time, that's just the nature of dark paint under direct sun. To actualy get it looking right again you'll want to polish it out once, then protect it and keep your new drying habits going. If it keeps eating at you, paint protection film on the worst panels is the only real escape hatch.

Agree with the blower advice, that was the single biggest upgrade to my routine. I did a full polish on my own black car two summers ago and since switching to air first, towel contact only, it took me about 14 months before I saw any new marks worth caring about. One thing I'd add, get a cheap handheld LED light and check the panels in your garage at night instead of in the sun. Direct sunlight exaggerates everthing and you'll drive yourself nuts chasing imperfections that a normal light barely shows. Polish once, then judge your technique under the LED, not the sun.

This is making me rethink my whole towel situation. I've been using the same batch of drying towels for like two years and they've been through the washer and dryer a bunch of times. I wonder if old crusty towels are half my problem even when I'm being gentle. Also that bit about misting the panel with a spray while it's wet, is that just any spray wax from the auto store or does it need to be somthing specific?

Update: so the towel age thing hit home, mine are years old and definitly crunchy. Tossed them and picked up fresh ones, plus started misting the panel before the final wipe like one of you mentioned. Big difference alredy, the long drag marks are way less obvious in the sun. Blower is next on the list.