Galvanized or copper in my opinion as a Master plumber who piped miles of compressed air lines.
Black will work fine but seems to create a lot of rust and debris as it ages and the moisture in the air corrodes it.
Most of the new systems, specifically designed for air, available today are pretty expensive and not easy to find parts. Read that as your not going to run down to the local hardware on Sat afternoon to get parts for repairs or additions. The other problem with "new" technology is it may not gain popularity and become obsolete.
To those that have used PVC pipe for compressed air and argue that "I haven't had a problem yet". That doesn't make it make sense.
As a young and dumb kid I used to run sleds in the winter and dirt bikes in the summer at high speed, WOT all the time. I had a 79 Honda Goldwing in excess of 110 mph on IA gravel roads. I had a Yamaha GP433 racing sled that would run 80 mph. " I did that all the time and haven't had a problem yet"
It took a buddy losing his life for me to stop that crap.