Black iron pipe is a poor choice for air lines. Its black because it has no coating. It is used for natural gas because gas has a detoriating effect on the zinc used to galvanize pipe which causes the zinc to come loose and end up in the gas stream. Little gas orifices don't like that at all.
The problem with black is that it rusts. Moisture in you air lines will rust the inside of the pipe and then you will be dealing with rust flakes entrained in your airstream. Compressed air and galvanized pipe seem to get along just fine. My personal chioce is copper pipe. It's light, can be assembled with nothing more than a propane torch, is very smooth inside, and helps cool the air off better.
I've had a couple of people say that the total installed cost is only a bit more expensice than other methods and you have a lifetime job.