I use air for winterizing everything, including the black tank flush. I set my compressor output for about 45 PSI, and this is both plenty of pressure AND it's not too much...which could cause damage to the plumbing. Note that when connected to city water, you use a pressure regulator adaptor to ensure that city water pressure won't blow up your RV's plumbing, too.
While the black tank flush SHOULD self drain, it takes only seconds to blow it out, so why take the risk of water being trapped somewhere, especially in the portion in the black tank?
For years I got away with using a pancake compressor for the task, but it couldn't supply continuous air at adequate pressure. I'd have to disconnect, allow the compressor to recharge, then connect again for a short "puff." A few years ago, when we began to use our rig during the winter on unseasonably warm weekends, I upgraded to this modest - but more than adequate - compressor: https://www.homedepot.com/p/Husky-30...303H/206532808
The key to this compressor's success in winterizing a camper is it's 30 gallon tank. The tank holds a LOT of air, and at 45 PSI, it can deliver all the CFM you need for the job essentially continuously. I'm very happy with this choice. And for the record, in my past life, I had a farm, and I had an 80 gallon, two cylinder, two stage compressor for the kinds of heavy work I did there, and yet this little Husky is impressive in its own right.
Set to the proper output air pressure (about 45 PSI), one can go through the rig turning faucets on and off repeatedly and even leave all faucets off for any amount of time with no fear of damaging the rig's plumbing.
Once the rig is blown dry and the grey tank is dumped into a bucket, I put about a gallon of RV antifreeze down the toilet, and about 2 gallons distributed to all interior drain traps...most of this goes into the grey tank.
Last tips on air winterizing...
1) be sure to FIRST drain and bypass the hot water heater before applying air. Compressed air can't deal with the hot water heater.
2) do each faucet individually...going from one to the next until no water spits out.
3) don't forget the inside tub/shower, outdoor shower, toilet flush, and low point drains, and the mentioned black tank flush.
4) don't forget to drain your fresh tank.
5) add RV antifreeze to all inside drains: kitchen sink, bathroom sink, tub/shower to fill the traps and and add enough to the grey tank to protect the drain valve.
6) add RV antifreeze down the toilet...almost a gallon...and then shut the flush valve and add about half a cup on top of the flush valve to keep things moist.
It took twice as long to write this as it does to perform a full winterization with air.