It is stainless steel
One procedure to blacken them was to paint them, and then bake the paint on in an oven
TM 9-1275 calls for a mixture of flat black paint thinned, 2 ounces thinner to 1 quart paint. Bake for 1.5 hours at 300 to 350*F. Gas cylinders should hang from a wire inside the oven, not lay flat on a tray
TM 9-1275 also mentions another manual, TM 9-1861, for another blackening procedure, but I don't have that manual